Article
pubs.acs.org/JPCA
Synthesis, Microwave Spectrum, Quantum Chemical Calculations,
and Conformational Composition of a Novel Primary Phosphine,
Cyclopropylethynylphosphine, (C3H5CCPH2)
Harald Møllendal,*,† Svein Samdal,† Jürgen Gauss,‡ and Jean-Claude Guillemin*,§
†
Centre for Theoretical and Computational Chemistry (CTCC), Department of Chemistry, University of Oslo, P.O. Box 1033,
Blindern, NO-0315 Oslo, Norway
‡
Institut für Physikalische Chemie, Universität Mainz, Duesbergweg 10-14, 55128 Mainz, Germany
§
Institut des Sciences Chimiques de Rennes, École Nationale Supérieure de Chimie de Rennes, CNRS, UMR 6226, 11 Allée de
Beaulieu, CS 50837 35708 Rennes Cedex 7, France
S Supporting Information
*
ABSTRACT: The microwave spectrum of cyclopropylethynylphosphine, C3H5CCPH2,
has been investigated in the 26−120 GHz spectral region. The spectrum is dominated by
very rich and complex a-type R-branch pile-ups. There must be insignificant steric
interaction between the phosphino group and the cyclopropyl ring due to the long distance
between these two groups. However, the phosphino group does not undergo free or nearly
free internal rotation. Instead, the spectra of two distinct conformers were assigned. Both
these two forms have CS symmetry. The symmetry plane bisects the cyclopropyl ring and the phosphino group in both
conformers, and the lone electron pair of the phosphino group points in opposite directions in the two rotamers. The energy
difference between the two forms was determined to be 1.9(6) kJ/mol. A simple model that takes into consideration the
interaction of the lone electron pair of the phosphino group with the π-electrons of the ethynyl group and the Walsh electrons of
the cyclopropyl ring is able to give a qualitative explanation of the observation of two conformers and the nonexistence of free
rotation of the phosphino group. The MW work was augmented by quantum chemical calculations using second-order Møller−
Plesset perturbation and coupled cluster theory with results that are in good agreement with the experiments.
■
or acetylenic silyl enol ethers28 or for many asymmetric
transformations in Rh-catalyzed hydrogenation and Rh- or Pdcatalyzed C−C bond-forming reactions,29 only a few primary or
secondary derivatives, which are kinetically unstable compounds, have been isolated by much more difficult syntheses.
Thus, the simplest ethynyl derivative HCC−PH2 was
prepared for the first time in 1987 by low-pressure electric
discharge of an acetylene−phosphine mixture.12 A few years
later, the chemoselective reduction of 1-alkynylphosphonates
was found to be a general approach for the low boiling
derivatives.30,31 Interestingly, 1-alkynylphosphines were found
to have much higher gas-phase acidities than the saturated
alkylphosphines,32 and their base-induced rearrangement
produced the corresponding phosphaalkynes.33
In this work, our studies of phosphines are extended to
include the synthesis and microwave study augmented with
quantum chemical calculations of the novel primary phosphine,
cyclopropylethynylphosphine (C3H5CCPH2). Several interesting physical properties are associated with this compound.
The distance between the phosphino group and the cyclopropyl ring is so long that steric interactions are hardly of
importance for the dynamical and conformational behavior of
INTRODUCTION
The conformational, structural, and dynamical properties of
many primary phosphines have been studied by microwave
(MW) spectroscopy in the past. These studies include
CH 3 PH 2 , 1 (CH 3 ) 2 PH, 2 , 3 (CH 3 ) 3 CPH 2 , 4 (CH 3 ) 3 P, 5
(CH 3 ) 2 CHPH 2 , 6 CH 3 CH 2 PH 2 , 7,8 cyclopropylphosphine
(C3H5PH2),9 phenylphosphine (C6H5PH2),10
H 2 PCH 2 CH 2 CN, 11 HCCPH 2 , 12 H 2 CCHPH 2 , 13,14
H2PCH2CH2PH2,15 HCCCH2PH2,16 H2CCHCH2PH2,17
H2CCCHPH2,18 phosphirane (C2H5P),19 cyclopropylmethylphosphine (C3H5CH2PH2),20 cyclopentadienylphosphine
(C5H5PH2),21 (chloromethyl)phosphine (ClCH2PH2),22 and
(2-chloroethyl)phosphine, (ClCH2CH2PH2).23 While one conformer exists for most of these phosphines, the existence of two
or more rotameric forms have been seen for each of
CH3CH2PH2,7,8 H2PCH2CH2CN,11 H2PCH2CH2PH2,15
H2CCHCH2PH2,17 H2CCCHPH2,24 C3H5CH2PH2,20
C5H5PH2,21 and ClCH2PH2.22 A comparison of the structural
and conformational properties of amines and phosphines has
recently been investigated in a gas electron-diffraction and
quantum chemical study,25 which concluded that these
properties are often different for phosphines and their amine
analogues.25
While many tertiary 1-alkynylphosphines have been synthesized26 and used as, for example, ligands in various reactions
like the gold-catalyzed cyclization of acetylenic β-ketoesters27
© 2014 American Chemical Society
Received: June 20, 2014
Revised: August 14, 2014
Published: August 29, 2014
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its phosphino group. This group could either rotate freely or a
barrier caused by electronic interactions of the lone electron
pair of the phosphorus atom with the ethynyl group π-electrons
and the Walsh electrons of the cyclopropyl ring could exist with
rotational isomerism as a consequence. The unknown dynamic
behavior of the phosphino group of C3H5CCPH2 was the
primary motivation to carry out the present research.
A successful investigation of the conformational and
dynamical problems presented by cyclopropylethynylphospnine
requires an experimental method with a superior accuracy and
resolution. Fortunately, MW spectroscopy meets these requirements and has therefore been chosen. The MW work is
augmented by high-level quantum chemical calculations, which
were conducted both to obtain information useful for the
assignment of the MW spectrum and for investigating the
conformational preferences in the case of C3H5CCPH2.
It was found in the course of this work that there is no free
rotation of the phosphino group and that two rotamers,
denoted I and II, exist instead. They are depicted in Figure 1,
Scheme 1Synthesis of Cyclopropylethynylphosphine
chloridic acid, diethyl ester (17.3 g, 0.1 mol) in THF (50 mL)
was cooled to −50 °C and the 2-cyclopropylethynyl
magnesium bromide solution was added dropwise. The mixture
was allowed to warm to room temperature and stirred for 15
min. The hydrolysis was then performed with a saturated
solution of NH4Cl (30 mL), and the organic compounds were
extracted with diethyl ether (3 × 50 mL). The solvents were
removed in vacuo and the cyclopropylethynylphosphonic acid,
diethyl ester was purified by distillation. bp0.1 = 105 °C. Yield:
11 g, 55%. 1H NMR (CDCl3, 400 MHz): δ 0.85 (m, 4H, CH2);
1.29 (t, 6H, 3JHH = 7.0 Hz, CH3); 1.33 (m, 1H, CH); 4.06
(quint, 3JHH = 3JPH = 7.0 Hz, 4H, OCH2). 13C NMR (CDCl3,
100 MHz): δ −0.25 (1JCH = 170.9 Hz (d), 3JCP = 5.1 Hz (d), cCH); 9.1 (1JCH = 165.8 Hz (t), 4JCP = 1.4 Hz (d), c-CH2); 16.1
(1JCH = 127.6 Hz (q), 3JCP = 6.3 Hz (d), CH3); 62.8 (1JCH =
146.7 Hz (t), 2JCP = 5.8 Hz (d), OCH2); 68.9 (1JCP = 305.2 Hz
(d), C−P); 105.9 (2JCP = 54.5 Hz (d), CCP). 31P NMR
(CDCl3, 160 MHz): δ −6.1. IR (film, cm−1) ν: 2986 (s), 2908
(m), 2201 (vs) (νCC), 1261 (vs), 1025 (vs), 840 (m).
Caution! Alkynylphosphines are pyrophoric and nauseating
compounds; preparation and handling must be carried out
under a well-ventilated hood.
Cyclopropylethynylphosphine. (For similar experiments,
see Guillemin et al.35) A vacuum line was equipped with two
traps. The first trap was immersed in a −60 °C cold bath, and
the second one was immersed in a −110 °C cold bath. A threenecked flask equipped with a septum, a stirring bar, and
containing the reducing mixture (50 mmol of AlHCl2 in 50 mL
tetraglyme) was attached to the vacuum line and degassed. The
cyclopropylethynylphosphonic acid, diethyl ester (1.0 g, 5
mmol diluted in 10 mL of tetraglyme) was slowly added (10
min) at room temperature with a flex-needle through the
septum. During and after the addition, the cyclopropylethynylphosphine was removed as it was formed from the reaction
mixture. The high boiling impurities were condensed in the first
trap, and the phosphine was selectively trapped in the second
one. At the end of the addition, the reaction mixture was still
stirred at room temperature for 20 min and then the trap was
disconnected from the vacuum line by stopcocks. Yield: 167
mg, 1.7 mmol, 34%. 1H NMR (CDCl3, 400 MHz): δ 0.65 (m,
2H, 1 H of each c-CH2); 0.73 (m, 2H, 1H of each c-CH2); 1.24
(m, 1H, c-CH); 3.60 (dd, 1JPH = 214.1 Hz, 4JHH = 2.2 Hz, 2H,
PH2). 13C NMR (CDCl3, 100 MHz): δ 0.41 (1JCH = 167.8 Hz
(d), c-CH); 7.9 (1JCH = 166.2 Hz (t), c-CH2); 60.6 (1JCP = 8.0
Hz (d), C−P); 108.1 (2JCP = 2.9 Hz (d), CC−P). 31P NMR
(CDCl3, 160 MHz): δ −177.1 [1JPH = 214.1 Hz (t)]. IR (gas,
cm−1): ν: 2969 (w), 2863 (w), 2296 (s) (νPH), 2198 (m)
(νCC), 1139 (s), 1077 (m), 830 (m).
Spectroscopic Experiments. The MW spectrum was
studied using the Stark microwave spectrometer of the
University of Oslo. Details of the construction and operation
of this device have been given elsewhere.36 Cyclopropylethy-
Figure 1. Models of conformers I and II whose MW spectra were
assigned. Atom numbering is given on I. The phosphino group has
different orientations in these two rotamers. Both conformers have CS
symmetry. The plane formed by the H8−C3−C9−C10−P11 chain of
atoms is the symmetry plane, which bisects the cyclopropyl ring and
the phosphino group in each conformer.
with atom numbering indicated on I. The lone pair (lp) of the
phosphorus atom and the C3−H8 bond are synperiplanar in I
and antiperiplanar in II. Both forms have CS symmetry. The
symmetry plane is formed by the H8−C3−C9C10−P11 link
of atoms. This plane bisects the cyclopropyl ring and the
phosphine group.
■
EXPERIMENTAL SECTION
Synthesis. The cyclopropylethynylphosphine was synthesized by chemoselective reduction of cyclopropylethynyldiethylphosphonate (Scheme 1). Cyclopropylethynyldiisopropylphosphonate had been prepared by Qu et al. by a different
approach.34
Cyclopropylethynylphosphonic Acid, Diethyl Ester.
Ethylmagnesium bromide (0.1 mol, 1 M) was prepared in THF
under dry nitrogen. In a flask equipped with a stirring bar and a
nitrogen inlet, cyclopropylethyne (6.6 g, 0.1 mol) in THF (50
mL) was cooled to −50 °C and the ethylmagnesium bromide
solution was added dropwise to form 2-cyclopropylethynyl
magnesium bromide. The mixture was allowed to warm to
room temperature and stirred for 15 min. In another flask
equipped with a stirring bar and a nitrogen inlet, phosphoro9420
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vibrational frequencies, quartic and sextic centrifugal distortion
constants,45 vibration−rotation constants (the α’s),46 and the re
and r0 rotational constants46 were computed observing the
precautions of McKean et al.47 These parameters are listed in
Tables 1S (conformer I) and 2S (rotamer II) of the Supporting
Information. The H8−C3···P11−H12 dihedral angle is 132.00°
in I and 311.95° in II. The lone electron pair of the phosphorus
atom is synperiplanar with the C3−H8 bond in I and
antiperiplanar in II. The MP2/cc-pVTZ electronic energy of
conformer I is 0.071 kJ/mol less than that of II. Corrected for
zero-point vibrational energies (ZPE) this difference becomes
0.029 kJ/mol. Moreover, the harmonic torsional frequencies of
the phosphino group are 59 cm−1 for I and 58 cm−1 for II, far
from free rotation. The transition state (TS), which has an
electronic energy of 2.19 kJ/mol (1.59 kJ/mol when corrected
for ZPE) above the energy of I, occurs at a value of 219.61° for
the H8−C3···P11−H12 dihedral angle. Its structure and
vibrational frequencies are listed in Table 3S of the Supporting
Information. The TS is depicted in Table 3S of the Supporting
Information.
The barrier height of 2.19 kJ/mol is low in comparison with
8.19 kJ/mol determined for the barrier to internal rotation in
CH3PH2.1 A reduction of the barrier from 8.19 kJ/mol of
methylphosphine was expected for cyclopropylethynylphosphine due to the fact that nonbonded interatomic distances are
much longer in the latter compound with insignificant steric
interactions and a lower barrier as a likely consequence.
MP2/cc-pVQZ calculations were performed to investigate
the dependency of the energy difference between I and II on
the basis set. It was assumed in these computations that the two
forms have a symmetry plane (CS symmetry). The results of
these calculations are shown in Tables 4S and 5S of the
Supporting Information. The electronic energy difference is
0.012 kJ/mol with I as the lower energy form compared to
0.071 kJ/mol found above in the MP2/cc-pVTZ result.
Similar calculations were performed at the CCSD/cc-pVTZ
(Tables 6S and 7S of the Supporting Information) and CCSD/
cc-pVQZ (Tables 8S and 9S of the Supporting Information)
levels of theory. Conformer I was found to be preferred by
0.042 kJ/mol in the former case, while II was computed to be
the more stable conformer by 0.005 kJ/mol in the latter case.
CCSD(T)/cc-pVTZ calculations (Tables 10S and 11S of the
Supporting Information) predict I to be 0.066 kJ/mol more
stable than II. It is concluded that all five theoretical methods
used in this work predict zero-energy differences between I and
II within methodological uncertainties.
Our experience is that CCSD/cc-pVQZ structures are often
more accurate than CCSD(T)/cc-pVTZ structures, and they
are therefore listed in Table 1 together with the dipole
moments and the electronic energy difference. The CCSD/ccpVQZ rotational constants calculated from these structures are
shown in Table 5 (conformer I) and in Table 6 (II) together
with their MP2 S reduction45 centrifugal distortion constants
and their experimental counterparts.
The CCSD/cc-pVQZ structures warrant discussion. A partial
r0 structure has been determined for HCCPH2. The value
given in this work12 for the C−P bond length is 177.4(5) pm,
very similar to the CCSD result, 177.1 pm (Table 1). The P−H
bond lengths of 141.3 (same table) compare well with the
reported 141.4(5) pm.12 A CC−P angle of 173(2)° was
required to obtain a consistent fit of the moments of inertia in
the case of HCCPH2.12 The corresponding angle is 185.2°
(−174.8°) for conformer I and 174.8 for II (Table 1). The
nylphosphine is a colorless liquid with a vapor pressure of
roughly 120 Pa at room temperature. Its spectrum was
recorded in the 26−120 GHz frequency interval at a pressure
of 5−10 Pa. A 2 m long Hewlett-Packard MW cell was
employed in the experiments, which were carried out with the
cell cooled by small portions of dry ice to about −30 °C. Radiofrequency microwave double-resonance experiments
(RFMWDR), similar to those of Wodarczyk and Wilson,37
were also conducted to unambiguously assign particular
transitions.
■
RESULTS
Quantum Chemical Methods. The ab initio calculations
were performed employing the Gaussian 09,38 Molpro,39 and
CFour40 programs. Frozen-core second-order Møller−Plesset
perturbation theory calculations41 (MP2), coupled cluster
calculations with single and double excitations42 (CCSD),
and CCSD calculations augmented by perturbative triples
corrections CCSD(T)43 were carried out. Dunning’s 44
correlation-consistent cc-pVTZ and cc-pVQZ basis sets were
used in the calculations. Default convergence criteria were used
in all calculations.
Computational Results. A MP2/cc-pVTZ potential
function for rotation about the C10−P11 bond was calculated
using the scan option of Gaussian 09. The H8−C3···P11−H12
dihedral angle, where the dots indicate that the phosphorus
atom P11 and the carbon atom C3 are not directly bonded to
one another, was scanned 360° in 10° intervals with no
restrictions on the variation of the remaining structural
parameters. The resulting function shown in Figure 2 has two
minima corresponding to conformers I and II separated by a
barrier of about 2 kJ/mol. This is a strong indication that there
is no free rotation of the phosphino group.
The MP2/cc-pVTZ structures of conformers I and II were
fully optimized and dipole moments, harmonic and anharmonic
Figure 2. Potential function of rotation about the C10−P11 bond.
Relative energies are given on the y axis and values of the H8−C3···
P11−H12 dihedral angle are shown on the x axis (see text). The dots
indicate that the phosphorus atom P11 and the carbon atom C3 are
not directly bonded to one another. This curve has minima at 132.00
(conformer I) and at 311.95° (rotamer II). The lone electron pair of
the phosphorus atom and the H8−C3 bond is synperiplanar in I and
antiperiplanar in II. The energy difference is 0.042 kJ/mol with I as the
lower-energy conformer. The transition state separating the two
conformers occurs at 219.61°, 2.19 kJ/mol above the energy of I.
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Table 1. CCSD/cc-pVQZ Structures, Dipole Moments, and Energy Difference of Conformers I and II of C3H5CCPH2
bond distance (pm)
conformer
C1−C2
C1−C3
C1−H4
C1−H5
C2−C3
C2−H6
C2−H7
C3−H8
C3−C9
C9−C10
C10−P11
P11−H12
P11−H13
I
angle (deg)
conformer
II
149.5
151.1
107.9
107.9
151.1
107.9
107.9
108.0
144.0
120.9
177.1
141.3
141.3
149.5
151.1
107.9
107.9
151.1
107.9
107.9
108.0
144.0
120.9
177.1
141.3
141.3
I
II
C2−C1−H4
C2−C1−H5
C3−C1−H4
C3−C1−H5
H4−C1−H5
C1−C2−H6
C1−C2−H7
C3−C2−H6
C3−C2−H7
H6−C2−H7
C1−C3−H8
C1−C3−C9
C2−C3−H8
C2−C3−C9
H8−C3−C9
C10−P11−H12
C10−P11−H13
118.4
117.9
116.9
116.7
115.4
118.4
117.9
116.9
116.7
115.4
116.4
119.1
116.4
119.1
115.2
97.7
97.7
118.4
117.9
116.9
116.7
115.4
118.4
117.9
116.9
116.7
115.4
116.4
119.1
116.4
119.1
115.2
97.8
97.8
I
H4−C1−C2−H6
H4−C1−C2−H7
H5−C1−C2−H6
H5−C1−C2−H7
H4−C1−C3−H8
H4−C1−C3−C9
H5−C1−C3−H8
H5−C1−C3−C9
H6−C2−C3−H8
H6−C2−C3−C9
H7−C2−C3−H8
H7−C2−C3−C9
C1−C3−P11−H12
C1−C3−P11−H13
C2−C3−P11−H12
C2−C3−P11−H13
H8−C3−P11−H12
H8−C3−P11−H13
dipole momentsa
μa
μb
μc
μtot
II
94.1
94.1
178.9
180.3
185.2
175.0
dihedral angle (deg)
conformer
angle (deg)
conformer
I
H12−P11−H13
C3−C9−C10
C9−C10−P11
0.0
−146.9
146.9
0.0
−2.4
142.7
−145.1
0.0
2.4
−142.7
145.1
0.0
−13.0
83.0
−83.0
13.0
132.0
−132.0
(10−30 C m)
4.76
0.0b
2.02
5.18
relative electronic energyb (kJ/mol)
0.005
II
0.0
−146.9
146.9
0.0
−2.5
142.7
−145.1
0.0
2.4
−142.7
145.1
0.0
165.7
−98.2
98.2
−165.9
−48.1
48.1
4.10
0.0b
1.79
4.48
0.0
1 Debye = 3.33564 × 10−30 C m. bRelative to conformer II. Total
electronic energy of II: −1405168.489 kJ/mol.
a
the two π bonds, provided that the symmetry of the substituent
is incompatible with the rotational symmetry of the triple bond.
The effect is rather small in case of substituents such as CH3
but can be rather pronounced in the case of substituents that
are either strong π donors or acceptors such as PH2, BH2, vinyl,
cyclopropyl, etc.
To explain the interactions in such monosubstituted
acetylenes, we assume that the substituent (D for π donor
and A for π acceptor) possesses just one orbital that can
interact with the π orbitals of acetylene. In the case of a donor
D, this orbital is occupied, and in the case of an acceptor A, it is
unoccupied. An orbital energy diagram for π interactions in
monosubstituted acetylenes is shown in Figure 3.
Assuming that these orbitals of D and/or A have πy
symmetry, this means that the πx and πx* orbitals of the
acetylene remain more or less unchanged, while there is a
substantial change in the πy and πy* orbitals. Most important is
probably here that a D substituent reduces the acceptor
capabilities of the πy* orbital, while at the same time this
substituent enhances the donor capabilities of the πy orbitals.
For an A substituent, the situation is reversed; this means that
the πy orbital now has an increased donor and the πy* a reduced
acceptor capability. The rather pronounced interaction of the
CC triple bond with these substituents results in a lengthening
C9C10 bond length is 120.9 pm compared to the
equilibrium bond length of acetylene, 120.2958(7) pm.48
Interestingly, the C−C bond lengths of the ring are different.
The C1−C2 bond length is 149.5 pm, whereas the C1−C3 and
C2−C3 bond lengths are 151.3 pm (Table 1). The CCSD/ccpVQZ C−C bond length in cyclopropane is 150.2 pm. The
lengthening of 1.1 pm of the C−C bonds adjacent to a
substituent and a shortening of the C−C bond opposite to the
substituent by 0.7 pm are in accord with predictions.49 The
equilibrium C−C bond length in cyclopropane50 is 150.30(10)
pm for comparison, which is the same within the uncertainty
limit as the CCSD/cc-pVQZ bond length (150.2 pm).
Existence of Distinct Conformers of Bisubstituted
Acetylenes. The existence of a barrier to internal rotation of
the phosphino group in cyclopropylethynylphosphine warrants
further investigation into the nature of this barrier. Acetylene
exhibits a triple bond, consisting of a σ bond and two equivalent
π bonds, and has an electron-density distribution that is
rotationally symmetric with respect to the CC bond axis. The
rotational symmetry might be broken in the case of
monosubstituted acetylenes, but this does not hamper the
free-rotation of the substituent. In other words, there exists no
distinct conformers and there is no rotational barrier
introduced. The substitution can only lift the degeneracy of
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πy orbitals (i.e., the same π orbitals as the first substituent or
those which are orthogonal to them). If the interactions with
both sets of π-orbitals are comparable, no distinct conformer
exists (apart from a rather small barrier) and the free rotation of
the substituent is still possible. However, if the interactions are
different, the free rotation is hindered and a specific orientation
is favored. Considering now the possibility of π-donors and
acceptors as substituents, the following conclusions can be
drawn: (a) in the case of D-CC-D′ (both substituents are
donors), an orthogonal arrangement is preferred, as the first
donor reduces the acceptor capabilities of the involved πorbitals. Therefore, the second donor, D′, interacts with the
second set of π-orbitals. (b) In the case of A-CC-A, everything
is reversed and the situation is more or less the same. (c) In the
case of A-CC-D, one should expect a parallel (and if different,
an antiparallel) arrangement, as both substituents interact with
the same set of π-orbitals.
Calculations for the choices A = BH2 and D = PH2 confirm
these expectations (see Table 3), as for both cases the parallel
conformations (and also antiparallel conformation in the case
of PH2) are no minima and only the orthogonal forms are the
minima. The barriers are 19.8 kJ/mol for BH2 and 2.5 kJ/mol
for PH2, sufficiently high to allow in principal an experimental
detection. On the contrary, for H2B−CC−PH2, one finds the
parallel arrangement to be the lowest in energy and the
orthogonal form to be a transition state.
With consideration now our system of interest, namely,
C3H5CCPH2 with PH2 and a cyclopropyl group as πdonating substituents, the simple model predicts that the lonepair orbital of PH2 and the three-membered ring, which defines
the direction for the π-donating Walsh orbital, are orthogonal
with respect to each other. There exist actually two possibilities
for realizing such an orthogonal orientation. In the first one, the
lone-pair of PH2 points in the same direction as the threemembered ring; in the second, it points in the opposite
direction. The transition state for interconversion between
these two conformers shows an arrangement with the
phosphorus lone pair parallel to the three-membered ring.
The computations, as well as the experiments, confirm these
expectations. Relevant results are listed in Table 4.
Microwave Spectrum and Assignments. An a−c
principal inertial axis plane bisecting the cyclopropyl ring and
the phosphino group is the symmetry plane in both
conformers. Both rotamers have their major dipole moment
component along the a-inertial axis calculated to be 4.76 and
4.10 × 10−30 C m for I and II, respectively (Table 1). There is a
smaller component along the c-inertial axis of 2.02 and 1.79 ×
10−30 C m for I and II (same table). μb is zero for symmetry
reasons in both cases. Ray’s asymmetry parameter51 κ is about
−0.998 for both forms. The microwave spectra of both I and II
were therefore predicted to consist of a comparatively strong
and predominating series of a-type R-branch pile-up regions
separated by almost exactly the sum of the B and C rotational
constants, as well as a much weaker c-type spectrum.
The CCSD value of B + C is 2015.1 MHz in the case of I
(Table 5) and 2038.8 MHz for II. The pile-ups belonging to
conformers I and II were therefore expected to occur close to
one another in the spectrum. Both conformers have several
low-frequency vibrations (Tables 1S and 2S of the Supporting
Information). In the case of I, the lowest MP2 harmonic
frequencies are 59, 104, 112, 254, 254, and 453 cm−1 (Table 1S
of the Supporting Information). Further harmonic fundamentals have frequencies above 500 cm−1. The lowest fundamental
Figure 3. Orbital energy diagram for π interactions in monosubstituted acetylenes.
of the CC bond length. In the same way, rather short bond
lengths to the substituents are observed. Substitution effects on
CC triple bond lengths of monosubstituted acetylenes are
demonstrated in Table 2, which is based on MP2 calculations.
Table 2. MP2/cc-pVTZ CC-Triple Bond Lengths in
Substituted Acetylenes
rCC (pm)
HCCH
HCCNH2
HCCCH3
HCC−cyclopropyl
HCC−vinyl
HCCPH2
HCCBH2
C3H5CCPH2 conformer I
C3H5CCPH2 conformer II
121.14
121.27
121.38
121.62
121.73
121.97
122.22
122.49
122.48
The π interactions between the CC triple bond and the
substituent can furthermore be visualized by investigating the
involved orbitals. The valence π orbitals are hence shown in
Figure 4 in the case of HCCPH2.
In the case of the bisubstituted acetylenes, the best approach
is to consider the interaction of the π-orbitals of the
monosubstituted acetylene with those of the second substituent. Two different possibilities now exist, namely, that the
π-orbital of the second substituents interacts with the πx or the
Figure 4. Valence π orbitals of HCCPH2.
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Table 3. Selected MP2/cc-pVTZ Results for Bisubstituted Acetylenes
type
orientation
ΔE (kJ mol−1)
rCC (pm)
lowest frequency (cm−1)
H2B−CC−BH2
AA
H2P−CC−PH2
DD
H2P−CC−BH2
DA
parallel
orthogonal
parallel
antiparallel
orthogonal
parallel
orthogonal
19.8
0.0
2.8
2.5
0.0
0.0
8.8
123.45
123.23
122.86
122.91
122.86
123.08
123.12
i297.6
153.4
i85.4
i87.8
87.8
144.1
i184.8
Table 6. Spectroscopic Constantsa of Conformer II of
C3H5CCPH2
Table 4. Selected MP2/cc-pVTZ Results for C3H5CPH2
conformer
orientation
ΔE
(kJ mol−1)
rCC
(pm)
lowest frequency
(cm−1)
I
II
TS
orthogonal
orthogonal
parallel
0.0
0.07
2.19
122.49
122.48
122.50
58.6
58.2
i59.2
experiment
vibrational state
ground
vibrationally excited
A (MHz)
B (MHz)
C (MHz)
DJ (kHz)
DJK (kHz)
DK (kHz)
d1 (kHz)
d2 (kHz)
HJ (Hz)
HJK (Hz)
HKJ (Hz)d
Ne
rmsf
12661(39)
1022.6670(17)
1014.8423(17)
0.106976(45)
−3.33545(98)
97.4c
0.00749c
0.000040c
0.0c
−0.00516(23)
0.0828(19)
939
0.829
12110(100)
1021.4243(21)
1013.7293(21)
0.105989(65)
−3.2268(15)
97.4c
0.00749c
0.000040c
0.0c
−0.00193(34)
0.0186(37)
598
0.791
Table 5. Spectroscopic Constantsa of Conformer I of
C3H5CCPH2
A (MHz)
B (MHz)
C (MHz)
DJ (kHz)
DJK (kHz)
DK (kHz)
d1 (kHz)
d2 (kHz)
HJ (Hz)
HJK (Hz)
HKJ (Hz)d
Ne
rmsf
experiment
theoryb
13069(23)
1014.4425(17)
1003.2655(17)
0.084213(47)
−2.1604(11)
86.9c
0.00475c
0.000025c
0.0c
−0.00433(21)
0.1294(18)
1011
0.954
13205.0
1013.2
1001.9
0.0816
−2.11
86.9
0.00475
0.000025
0.000055
−0.0328
1.81
theoryb
12714.9
1020.6
1012.2
0.104
−3.33
97.4
0.00749
0.000040
0.000086
−0.043
2.18
a
S-reduction, Ir-representation.45 Uncertainties represent one standard
deviation. The spectra are listed in Tables 13S and 14S of the
Supporting Information. bComments as in Table 5. cComments as in
Table 5. dComments as in Table 5. eComments as in Table 5.
f
Comments as in Table 5.
a
S-reduction, Ir-representation.45 Uncertainties represent one standard
deviation. The spectrum is listed in Table 12S of the Supporting
Information. bCCSD/cc-pVQZ rotational and MP2/cc-pVTZ centrifugal distortion constants. cFixed. dRemaining sextic centrifugal
distortion constants preset at zero in the least-squares fit. eNumber of
transitions used in the fit. fRoot-mean-square deviation defined as rms2
= Σ[(νobs − νcalc)/u]2/(N − P), where νobs and νcalc are the observed
and calculated frequencies, u is the uncertainty of the observed
frequency, N is the number of transitions used in the least-squares fit,
and P is the number of spectroscopic constants used in the fit.
K−1. These splittings, which also lead to a reduction of
intensity, can be comparatively large and, moreover, were
predicted to lead to frequent overlapping of spectral lines
originating from other excited vibrational states, which is yet
another complicating factor.
Survey spectra revealed the expected patterns of extremely
dense and relatively weak aR-pile-ups. Figure 5 shows a 140
MHz portion of the J = 31 ← 30 pile-up region with its
characteristic spectral density. This region is actually about 1
GHz wide and encompasses the ground and many vibrationally
excited states of both I and II. Most of the transitions of the
band head of this region belong to the ground state of I. The
pile-ups cover larger and larger frequency intervals as J
increases. For J larger than about 40, a continuous spectrum
was observed.
The first assignments were obtained using the aR-pile-up
transitions. The assignment of the J quantum numbers
associated with each pile-up was obvious. The assignments of
the K−1 pairs were much less obvious. Fortunately, the MP2
centrifugal distortion constants were very helpful to obtain
correct assignments of them. The fact that these K−1-pairs are
modulated at very low Stark fields was another useful property
that was exploited for this purpose.
The spectrum of the ground vibrational state of conformer I
was first assigned. The strongest lines of this state are located in
the band heads of the pile-ups (see Figure 5). The assignments
started with the K−1 > 4 pairs and were gradually extended to
higher and higher values of J and K−1. The K−1 = 1 and 0 were
vibration (59 cm−1) is the torsional vibration, whereas the
remaining four vibrations below 500 cm−1 are bending
vibrations. Similar results were obtained for II (Table 2S of
the Supporting Information). The Boltzmann populations of
vibrationally excited states of these five low-frequency
fundamentals are significant at the recording temperature of
−30 °C. This reduces the intensity of the spectrum and at the
same time produces very complicated and dense patterns of
vibrationally exited-state spectra.
The intensity of the transitions increases with the square of
the frequency, and it was predicted that most assignments had
to be made for high values of J in the upper part of the 26−120
GHz spectral region due to low intensities of the spectral lines.
Centrifugal distortion becomes increasingly important as J and
K−1 increase. While the K−1 < 3 or 4 transitions are split by
asymmetry, the MP2 centrifugal distortion constants (Tables 5
and 6) predict that higher K−1-states are generally split by
centrifugal distortion into pairs of lines having the same value of
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One vibrationally excited state, which is assumed to be the
first excited state of the torsion about the C−P bond or a low
bending vibration was also assigned. The spectrum consisting
of 598 transitions are shown in Table 14S of the Supporting
Information, while the spectroscopic constants are displayed in
Table 6.
The spectroscopic constants in Tables 5 (conformer I) and 6
(conformer II) merit discussion. The A rotational constants are
poorly determined, which is due to the fact that only aR-type
transitions have been assigned for these highly prolate
compounds, and these constants are therefore not discussed
further. Very accurate values have been found for B and C. The
MP2 calculations (Tables 1S and 2S of the Supporting
Information) predict that the experimental effective B and C
rotational constants should be approximately 3 MHz smaller
than the equilibrium B and C constants. The CCSD/cc-pVQZ
B and C rotational constants, which have been calculated from
approximate equilibrium structures (Table 1), are smaller by
1−2 MHz than the experimental constants, whereas the
opposite would presumably have been the case for rotational
constants derived from true equilibrium structures. This
indicates that the CCSD/cc-pVQZ structures are close to the
equilibrium structures, but calculations at even higher
methodological levels are needed to obtain the true equilibrium
structures.
Accurate values could be obtained only for two quartic
centrifugal distortion constants, namely DJ and DJK, while the
remaining three constants DK, d1, and d2 were preset at the
MP2 values in the fitting of the spectra. Attempts to determine
d1 and d2 were made, but only very uncertain results were
obtained. DK could not be determined at all. The problems
encountered for these three centrifugal distortion constants are
natural since I and II are practically prolate rotors and only aRtransitions have been available for their eventual determination.
Interestingly, the MP2 values of DJ and DJK are very close to
their experimental equivalents in Tables 5 and 6.
Two sextic centrifugal distortion constants, HJK and HKJ,
were determined with the remaining sextic constants preset at
zero. The MP2 values of these two constants are listed in
Tables 5 and 6 together with their experimental counterparts.
The agreement between experiment and theory is poor in these
cases. The reason could be that the derivation of the sextic
constants are too demanding for computations at the MP2 level
of theory.
Internal Energy Difference. Comparison of intensities of
carefully selected transitions was used to derive the internal
energy differences between the ground vibrational states of I
and II. A variant of eq (3) of Esbitt and Wilson53 (see also
Townes and Schawlow54) was employed to calculate the energy
difference from the spectral intensities. In accordance with
Esbitt and Wilson,53 the energy difference E″ − E′ between two
conformers is given by
Figure 5. A 140 MHz portion of the MW spectrum taken at a Stark
field strength of about 110 V/cm. The J = 31 ← 30 a-type transitions
occur in this region. This spectral interval is actually about 1 GHz wide
and includes a very large number of transitions of the ground and
vibrationally excited states of both I and II. The strong transitions at
the band head (left) belong to the ground vibrational state of
conformer I. The intensity is given in arbitrary units.
finally assigned by means of their Stark effects and spectral
positions. The assignments of several K−1 = 3 transitions were
confirmed by RFMWDR experiments. The spectrum consisting
of 1011 aR-transitions with J between 13 and 59 and with K−1
up to 26 is listed in Table 12S of the Supporting Information.
The transitions were least-squares fitted to Watson’s S-reduced
Hamiltonian45 using Sørensen’s program Rotfit,52 and the
spectroscopic constants shown in Table 5 were obtained. The ctype spectrum was predicted using these constants, but
extensive searches for it were futile. MW intensities are
proportional to the square of the dipole moment components.
The dipole moment components of Table 1 indicate that the
a
R-transitions are much stronger, at least by a factor of 5−6,
than the c-type transitions. The observed a-type spectrum is
comparatively weak, and the c-type spectrum would be much
weaker and was therefore not possible to assign its transitions.
Most vibrationally excited state spectra of I are located at
higher frequencies than their ground-state counterparts. The
spectrum of a prominent vibrationally excited state of I was
observed. Unfortunately, it was not possible to get detailed,
unambiguous assignments of the K−1 lines due to extensive
overlapping by the spectra of other vibrationally excited states.
The value of B + C is 2022.2 MHz for this state, which is
possibly the first excited state of the phosphino group torsion.
The changes in the CCSD rotational constants from I to II
were used together with the experimental rotational constants
of I (Table 5) to predict the approximate frequencies of aRtransitions belonging to conformer II. These transitions, which
appear at higher frequencies than their conformer I counterparts, are overlapped by several vibrationally excited states
presumably belonging both to I and II. This made the
assignment of the aR-spectrum more difficult than in the case of
I. A total of 939 transitions with Jmax = 57 and K−1max = 27 listed
in Table 13S of the Supporting Information were ultimately
used to determine the spectroscopic constants shown in Table
6. Searches for c-type lines were futile for the same reasons as
those discussed above for conformer I.
E″ − E′ = RT ln L
(1)
where E′ and E″ are the internal energies of the two conformers
in their ground vibrational states, R is the universal gas
constant, and T is the absolute temperature. L is given by
L=
2
S′ g ″ ⎛ v″μ″ ⎞ l″ Δv′ λ″ ⎛ 2J ′ + 1 ⎞
⎜
⎟
⎜
⎟
S″ g ′ ⎝ v′μ′ ⎠ l′ Δv″ λ′ ⎝ 2J ″ + 1 ⎠
(2)
where S is the peak signal amplitude of the radiationunsaturated line, g is the degeneracy other than the rotational
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degeneracy, which is 2J + 1, v is the frequency of the microwave
transition, μ is the principal-axis dipole moment component, l is
the radiation wavelength in the Stark cell, Δv is the line breadth
at half height, λ is the line strength, and J is the principal
rotational quantum number.
The radiation wavelength (l) and the degeneracy (g) were
assumed to be the same for the two transitions whose
intensities were compared. The CCSD/cc-pVQZ a-axis dipole
moment components (μ) were employed in the calculations of
the energy differences because they have not been determined
experimentally. It was especially difficult to measure accurately
the peak signal amplitudes (S) and the line breadths (Δv)
because the lines are relatively weak and the spectrum is very
crowded, resulting in frequent overlaps of spectral lines and
Stark lobes.
The four transition pairs used to determine the energy
difference are listed in Table 14S of the Supporting
Information. All these transitions have relatively rapid Stark
effects and are fully modulated at comparatively low Stark
fields. They also appear to be well-separated from other lines. It
is seen from Table 14S of the Supporting Information that the
energy differences derived from the different pairs vary between
2.2 and 1.5 kJ/mol. The average energy difference is 1.9 kJ/mol
with II as the lowest-energy form. It is difficult to estimate one
standard deviation to this number, but 0.6 kJ/mol seems
reasonable considering the many factors that contribute to the
uncertainty in this case.
The experimental one standard deviation of 0.6 kJ/mol
means that the 95% confidence interval is ±1.2 kJ/mol, which
suggests that the two rotamers may be close to equal in energy.
This is in accord with the theoretical energy differences that
were zero within the methodological uncertainties (see above).
However, II is found to be slightly lower in energy than I in the
most advanced calculations (CCSD/cc-pVQZ). It should be
pointed out that the energy difference (0.005 kJ/mol) obtained
in the CCSD/cc-pVQZ calculations is not the same as the
experimental difference [1.9(6) kJ/mol]. The theoretical
energies refer to the approximate equilibrium structures of I
and II, whereas the experimental values refer to the internal
energies of the ground vibrational states of the two rotamers.
distortion constants, rotation−vibration constants, and differences between ground-state and equilibrium rotational
constants. Microwave spectra of the ground and vibrationally
excited states of two conformers. Transition pairs used in
intensity measurements. This material is available free of charge
via the Internet at http://pubs.acs.org.
■
*E-mail: harald.mollendal@kjemi.uio.no. Tel: +47 2285 5674.
Fax: +47 2285 5441.
*E-mail: jean-claude.guillemin@ensc-rennes.fr. Tel:
+33223238073.
Notes
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS
We thank Anne Horn for her skillful assistance and Celine
Levron for recording infrared spectra. This work has been
supported by the Research Council of Norway through a
Centre of Excellence Grant (Grant 179568/V30). It has also
received support from the Norwegian Supercomputing
Program (NOTUR) through a grant of computer time
(Grant NN4654K). J-C.G. thanks the Centre National d’Etudes
Spatiales (CNES) for financial support. The work in Mainz was
supported by the Deutsche Forschungsgemeinschaft (DFG GA
370/5-1). J.G. also thanks the Center of Theoretical and
Computational Chemistry (CTCC) at the University of Oslo
for the hospitality and financial support during a sabbatical.
■
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■
CONCLUSIONS
The phosphino group is situated far away from the cyclopropyl
ring in C3H5CCPH2. One might therefore suspect that this
group would undergo practically free rotation. Quantum
chemical calculations and the MW spectrum show that this is
not the case. Instead, two rotameric forms separated by a
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■
AUTHOR INFORMATION
Corresponding Authors
ASSOCIATED CONTENT
S Supporting Information
*
Results of the theoretical calculations, including electronic
energies, molecular structures, dipole moments, harmonic and
anharmonic vibrational frequencies, rotational and centrifugal
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