Combined Photoelectron, Collision-Induced Dissociation, and
Computational Studies of Parent and Fragment Anions of
N-paranitrophenylsulfonylalanine and N-paranitrophenylalanine
Jason Lambert,a Jing Chen,b Angela Buonaugurio,b Kit H. Bowen,b Chi-Linh DoThanh,cYilinWang,c Michael D. Best,c R. N. Compton,b,c Thomas Sommerfeldd
a
Department of Physics & Astronomy, University of Tennessee, Knoxville, TN 37996, USA
b
Department of Chemistry, Johns Hopkins University, Baltimore, Maryland 21218, USA-
c
Department of Chemistry, University of Tennessee, Knoxville, TN 37996, USA
d
Department of Chemistry & Physics, Southeastern Louisiana University, Hammond, Louisiana
70402, USA
Supplemental Material for paper.
1. Methods for computing the AEA, VDE, and VEA associated with the 2B1 valence
state of the nitrobenzene anion
Nitrobenzene is a useful reference system for NPNPSA and NPNPA, because both of the
larger molecules form analogous valence anion states where the excess electron is essentially
localized on the nitrobenzene moiety. Moreover, the experimental AEA of nitrobenzene of 1.00
eV,1 is well known. However, the experimental result includes zero-point effects and for
validation purposes, the AEA value without corrections is more useful. To obtain a zero-pointeffect-free “experimental” value, the zero-point correction of the AEA of nitrobenzene was
computed using three density functionals, M06-2X, B3LYP, and TPSS, with the aug-cc-pVDZ
basis set. The three functionals agreed with each other reasonably well and yielded corrections of
84, 71, and 73 meV, respectively. Therefore, the experimental value for the purely electronic
AEA of nitrobenzene should be close to 0.92 eV, and this is the value we will subsequently
compare with.
The first set of methods considered is the usual sequence of ab initio methods, SCF, MP2
and coupled-cluster based methods. Unfortunately, the UHF wavefunction for the anion shows a
large spin-contamination, so that neither SCF nor MP2 are expected to yield useful results,
which is indeed the case (see Table S1). Coupled cluster calculations are known to be able to
recover from spin contamination in the reference wavefunction, and UHF-based CCSD(T)
calculations do yield results much more reasonable than UHF or UHF-based MP2. Yet,
CCSD(T) calculations started from a ROHF reference, or from a reference constructed using the
orbitals of the neutral (QRHF) yield virtually identical results, which are quite a bit higher than
the UHF based coupled-cluster results (~70 meV) suggesting that in this case recovery from
spin-contamination is not complete. Another test is direct computation of the electron affinity
with the EOM-CCSD method, which gives results in close agreement with ROHF-based
CCSD(T). Regarding basis sets effects, the coupled cluster values follow the usual trend that
the experimental value is approached from below reflecting that there is more electron
correlation in the anion than there is in the neutral. With the aug-cc-pVDZ basis set the result is
about 0.1 eV too low, with the aug-cc-pVTZ set this difference is down to less than 50 meV, and
thus the CCSD(T) calculations clearly approach the “experimental” value of 0.92 eV is a
systematic manner. Based on this finding, the ROHF-based CCSD(T) and EOM-CCSD values
for the VDE and VEA are also expected to represent good guidelines for the validation of density
functional methods.
Eight different density functionals were used to compute the VEA, VDE, and AEA of
nitrobenzene, the M06-2X, B3LYP, and O3LYP hybrid functionals, the less expensive metaGGA functional TPSS, and the GGA functionals BLYP, BP86, PBE, and OLYP. All functionals
systematically over-bind the electron with the most recently developed functional used, M06-2X,
doing in fact the worst job at predicting the respective detachment and attachment energies. All
density functional methods show the trend that the experimental value is approached from above
as the basis set is expanded, and even with the aug-cc-pVTZ set, most functionals over-bind by
more than 0.1 eV. The exception is the OLYP GGA, which yields AEAs in perfect agreement
with the experimental AEA if the aug-cc-pVTZ basis set is used. The situation is worse for the
VDE and even more so for the VAE. The formerly more reliable hybrid functionals tend to overbind by 0.3 eV or more, while the GGAs do not show a clear trend. In particular OLYP, and to
some extend the BLYP and TPSS functionals, perform significantly better than the hybrid
functionals, a trend that has been noticed before but even those two functionals significantly
overestimate the VAE of the 2B1 state.
Since density functional results tend to be less dependent on diffuse functions than ab
initio methods and this is in particular true for GGAs, a triple-z set augmented with a diffuse spset on the heavy atoms and a single diffuse s function on H was considered.44 Indeed using
Ahlrich's Def2-TZVP (ma-Def2-TZVP) set with this minimal augmentation yields results in very
close agreement with the aug-cc-pVTZ results for both the OLYP and TPSS functionals.
In conclusion, CCSD(T) or EOM-CCSD are as expected needed to compute reliable
electron binding energies, yet, for systems larger than nitrobenzene these methods are not
practicable even with the “minimal” aug-cc-pVDZ set. In the nitrobenzene case ΔMP2 is not an
alternative, because of the large spin-contamination in the unrestricted SCF calculation for the
anion, and the only alternative are density functional based methods. Regarding the geometry
optimizations, hybrid functionals such as B3LYP, or better M06-2X, are anyway expected to be
the best tradeoff between reliability and computational cost. However, our results show that
these functionals are not the best choice for computing electron attachment energies, but that the
OLYP GGA functional, and to some extend the TPSS and BLYP functional yields results in
better agreement with CCSD(T) and experiment. Based on our results one may expect that for
systems similar to nitrobenzene AEAs of will be well reproduced if triple- basis sets are
employed, while VDEs and VEAs are expected to be overestimated by a few tenth on an eV.
Table S1. Ab initio and DFT calculations for the EBEs (in eV) of the 2B1 valence state of
nitrobenzene.
SCF
MP2
UHFCCSD
UHFCCSD(T)
ROHFCCSD(T)
ORHFCCSD(T)
EOMCCSD
M06-2x
B3LYP
O3LYP
BLYP
BP86
PBE
OLYP
TPSS
AugpVDZ
VAE
0.31
-0.81
VDE
1.33
-0.25
0.33
1.08
0.26
0.92
0.77
0.31
0.98
0.83
0.31
0.98
0.83
0.25
0.95
0.70
0.69
0.73
0.65
0.84
0.76
0.57
0.66
1.44
1.33
1.34
1.20
1.40
1.31
1.13
1.23
Aug-pVTZ
AEA
VAE
VDE
AEA
1.21
1.05
0.81
0.45
1.12
0.88
0.79
0.42
1.12
0.88
1.08
1.12
1.05
1.15
1.30
1.19
0.97
1.12
0.78
0.71
0.75
0.63
0.83
0.75
0.57
0.65
1.53
1.35
1.36
1.19
1.39
1.30
1.12
1.23
1.12
1.08
1.03
1.08
1.23
1.13
0.92
1.06
2. Methods for computing the VEA associated with the Dipole Bound state of the
nitrobenzene anion
Computing attachment energies associated with dipole-bound states is far more
challenging than those with valence states. First, large sets of additional diffuse functions are
needed, and saturating the basis set in this respect is critically important. Second, DFT methods
cannot be used in the same straightforward way as for valence bound states. On the one hand,
dipole-bound states are very diffuse, and therefore the long-range part of the functional must be
free of electron self-interaction, so that the asymptotic electron-molecule interaction is correct.
This is a principle problem, and uncorrected functionals such as B3LYP will give infinite
binding energies in the complete basis set limit. On the other hand, there is the practical problem
of choosing a suitable integration grid for Gaussian functions with very small exponents. In fact,
in many programs it becomes practically impossible to reach convergence for the Kohn-Sham
iterations, if three or four sets of progressively more diffuse functions are added to the basis set.
Last, the attachment energies of dipole-bound states are very sensitive to long-range electron
correlations effects, and reliable results often require CCSD(T) calculations with large valence
basis sets.34 Nitrobenzene is to some extent a typical case. On an absolute scale the VEA of the
dipole-bound state nitrobenzene is small, in the order of 15 meV, and Koopmans's Theorem as
well as ΔSCF and ΔMP2 seem to be fairly close, but in relative terms the deviations are of cause
significant. The often reliable, and more importantly robust direct methods EOM-MP2 and
EOM-CCSD, overestimate the VEA making nitrobenzene a particular challenging case. For
larger systems, such as NPNPSA or NPNPA, computing VEAs with CCSD(T) and triple-ζ
quality basis sets is clearly out of question, and ΔMP2 seems to be the most promising tradeoff
for larger system. Unfortunately, this tradeoff is partly spoiled by the presence of nearby valence
states, which are easily separated by symmetry for nitrobenzene, but which cannot be separated
for most conformers of NPNPSA or NPNPA.
Table S2. Calculations for the VEAs (in meV) of the dipole-bound state of nitrobenzene.
KT
DSCF
DMP2
DCCSD
DCCSD(T)
EOM-MP2
EOM-CCSD
Aug-cc-pVDZ+6s6p5d
6.4
7.1
8.9
16.6
15.6
29.9
23.5
Aug-cc-pVTZ+6s6p5d
6.3
24.6
Table S3. Relative energies of conformers of neutral NPNPA. Minimal energy structures have
been computed using M06-2X/aug-cc-pVDZ; the MP2/aug-cc-pVDZ energies have been
evaluated at these geometries. All Cartesian coordinates are found in the supplementary data.
Conformer (Neutral)
1
2
3
4
5
6
M06-2X Energy (kJ/mol)
0
36.9
10.9
28.5
41.3
16.8
MP2 Energy (kJ/mol)
0
30.6
2.7
22
36.6
9
Table S4. Relative energies of conformers of neutral NPNPSA. Minimal energy structures have
been computed using M06-2X/aug-cc-pVDZ; the MP2/aug-cc-pVDZ energies have been
evaluated at these geometries. All Cartesian coordinates are found in the supplementary data.
Conformer (Neutral)
1
2
3
4
M06-2X Energy (kJ/mol)
21.6
0.0
13.1
9.3
MP2 Energy (kJ/mol)
19.8
0.0
13.2
9.1
Table S5. Relative energies of conformers of the NPNPA valence anion. The aug-cc-pVDZ
basis set has been used. All Cartesian coordinates are found in the supplementary data.
Conformer (Anion)
123456-
M06-2X Energy (kJ/mol)
28.1
6.3
0
13.3
32.7
6.6
Table S6. Relative energies of conformers of the NPNPSA valence anion. The aug-cc-pVDZ
basis set has been used. All Cartesian coordinates are found in the supplementary data.
Conformer (Anion)
1234-
M06-2X Energy (kJ/mol)
21.6
29.5
4.5
0.0
Table S7. Dipole moment of NPNPA conformers and VEA associated with NPNPA dipolebound states. The dipole moments have been computed using the M06-2X functional, and the
VEA of the dipole-bound states has been computed at the M06-2X geometries, using KT, ΔSCF,
and ΔMP2 with the aug-cc-pVDZ set further augmented with a 6s6p5s set centered at the center
of mass of the molecule.
VEA
VEA
VEA
KT[meV]
ΔSCF[meV]
ΔMP2[meV]
dipole moment
Conformer
[Debye]
1
7.6
20
22.6
36.5
2
2.9
2
2.4
11.2
3
7
54.2
67.3
106.6
4
8.5
58.6
70.4
111.5
5
8.5
58.7
68.6
103.1
6
6.9
65
84.4
141.3
Table S8. Calculated changes in Gibbs Free Energy (ΔG) and Enthalpy (ΔH) of NPNPSA-H
with B3LYP/aug-cc-pVDZ, including vibrational corrections.
Products
273→201+72
273→186+44+43
273→138+135
273→122+151
ΔG (eV)
1.52
-0.53
-1.06
3.20
ΔH (eV)
2.05
-1.57
-0.54
3.73
Table S9. Calculated changes in Gibbs Free Energy (ΔG) and Enthalpy (ΔH) of NPNPA-H with
B3LYP/cug-cc-pVDZ, including vibrational corrections.
Products
209 →165+44
209 →150+59
209 →123+42+44
209 →122+43+44
ΔG (eV)
1.43
4.21
1.53
1.95
ΔH (eV)
1.92
4.92
2.58
2.97
Figure S1. The CID spectrum of deprotonated NPNPSA.
Figure S2. The CID spectrum of deprotonated NPNPA.
Figure S3. Optimized structure of neutral 87 amu fragment for both NPNPA and NPNPSA. The
CO2 was originally bound to the carbon bound to the nitrogen.