Infrared spectroscopic investigation
on high acidity of diethylether cation
Tomoya Endo1, Yoshiyuki Matsuda1, Asuka Fujii1,
and Kaito Takahashi2
1
Department of Chemistry, Graduate
School of Science, Tohoku University,
Japan
2 Institute of Atomic and Molecular Sciences,
Academia Sinica, Taiwan
Introduction
OH・・・X, NH・・・X
Cationic OH and NH are highly acidic, so that they act as strong
hydrogen bond donors.
Structures of (NH3)2+ & (CH3OH)2+
(CH3OH)2+
(NH3)2+
H+-transferred
structure
JCP 125, 164320 (2006).
H+-transferred
structure
JCP 129, 094306 (2008).
Formed through barrierless H+-transfer from NH or OH
Cationic NH and OH are highly acidic
CH・・・X
Neutral alkyl groups generally do not act as strong proton donor.
Is a proton of CH (alkyl group) acidic?
Diethylether (DEE) cation
Proton-transfer in the VUV photoionization of diethylether dimer
VUV
ionization
Proton-transfer
from CH
(DEE)2
(DEE)2+
After the photoionization of (DEE)2, proton transfer from CH occurs
without the energy barrier.
CH of DEE+ is highly acidic.
This study
Aim
Diethylether (DEE) cation
Understand the origin of the high acidity of
diethylether (DEE) cation
Infrared spectroscopy and theoretical calculation of DEE+
1) IR spectroscopy of DEE monomer cation with the VUV
photoionization detection @Sendai, Japan
2) Optimizations of structures, normal vibrational calculations,
and NBO calculations of the charge distribution
and hyperconjugation (mainly perfromed @IAMS, Taiwan)
Experiment
IR Predissociation Spectroscopy of VUV-Pumped Ion (IRPDS-VUV-PI)
Matsuda et al. PCCP 11, 1279 (2009).
dissociation
V’
IR
1st. Q-mass
2nd. Q-mass
IR
IE
Dt
VUV
ion signal
VUV
octopole
ion guide
channeltron
detector
rare gas
cell
S0
・Molecules or clusters in supersonic jet are ionized with VUV(@118nm).
・The cation under investigation is size-selected with the 1st Q-mass.
・We monitored fragment ions generated with IR predissociation of the
cation through the 2nd Q-mass.
We can observe IR spectra of size-selected cations generated by
VUV photoionization
IR spectra of DEE+ and DEE+-Ar
?
Cations are generated
@118 nm
DEE+
Lowering
Internal temperature
DEE+-Ar
2400
2600
2800
3000
3200
Wavenumber / cm-1
3400
3600
3800
Intense broad band out of the normal CH stretch vibrational region
Narrowing and low frequency shift through Ar attachment
IR spectra of DEE+ and DEE+-Ar
?
Cations are
generated
@118 nm
DEE+
Lowering
Internal temperature
DEE+-Ar
0 kJ/mol
nCH
2400
2600
scaled by 0.96
2800
3000
3200
3400
Wavenumber / cm-1
3600
3800
1.8 kJ/mol
B3LYP/6-31++G(d,p)
The band at 2700 cm-1 is assigned to nCH next to the O atom
The frequency & intensity of the CH band depend on
internal rotation of -CH2CH3
Internal rotation dependence of nCH frequency & Intensity
nCH
nCH frequency
2900
2800
2700
nCH intensity (km/mol)
nCH frequency/ cm-1
3000
200
nCH intensity
2600
150
Geometry optimizations and vibrational calculations
with changing C1O2C3H4 dihedral angle.
100
50
0
0
50 100 150 200 250 300 350
Internal rotational angle of the ethyl group
(C1O2C3H4 dihedral angle) / degree
scaled by 0.96
B3LYP/6-31++G**
120°
240°
CH stretch frequency changes
from 2640 cm-1 to 2940 cm-1 with
the rotation of the ethyl group.
The intensity increases with the
low frequency shift of the band.
360°
( 0°)
Why do the frequency and intensity depend on the
rotational angle of the ethyl group?
Hyperconjugation of CH and O atom
Stabilization energy by hyperconjugation of s orbital (CH) and SOMO (O)
Stabilization energy through
delocolization of s eletron
(CH) to SOMO (O atom)
kcal/mol
30
sCH to SOMO (O)
15
0
0
60
120
180
240
300
Dihedral angle of COCH / degree
360
The degree of hyperconjugation depends largely on the internal
rotational angle of the ethyl group.
Delocalization of the s electron of CH to SOMO of oxygen (hyperconjugation)
Lower frequency shift and increase of intensity of CH stretch
The high acidity of the CH bond through hyperconjugation
Hyperconjugation between the s orbital and SOMO of the O atom
Delocalization of the s electron
Increase of acidity of CH and weakening the CH bond
sCH to SOMO (O)
Degree of hyperconjugation
High
Low
Acidity of CH bond
Hyperconjugation strongly depends on the COCH dihedral angle
Acidity of CH depends highly on the orientation of
the ethyl group
Summary
1) We observed the IR spectra of DEE cation with spectroscopy based on
the VUV photoionization detection. A CH stretch is observed at 2700
cm-1 with large bandwidth and high intensity.
2) The frequency, intensity, and large bandwidth of the CH stretch at 2700
cm-1 originates from the hyperconjugation between the s orbital and
SOMO of the oxygen atom. The hyperconjugation largely depends on
the internal rotation of the ethyl group.
3) The high acidity of DEE cation originates from the hyperconjugation,
and thus the acidity of DEE cation largely changes with the internal
rotation of the ethyl group.
Thank you for your attention!
The intensity and acidity of the CH
The increase in the intensity of the
CH stretch band
0.30
200
0.25
intensity of nCH
150
100
0.20
50
The intensity of nCH (km/mol)
Natural atomic charge (atomic unit)
charge of H
0
0
50 100 150 200 250 300 350
Dihedral angle of C4O3C2H6/ degree
The increase in the positive charge
(acidity) of the H atom of the CH
J. Mol. Struct. 113, 323 (1984). J. Mol. Struct.
224 363 (1990). J. Mol. Struct. 521, 1 (2000) etc.
High acidity
CH3CHO
protic
Intensities of the CH
stretch bands of a
variety of neutral
molecules
J. Mol. Struct. Low
521, 1 (2000）etc.
acidity
aprotic
B3LYP/6-31++G**
The transition intensity changes with the difference in the
acidity of the CH.
The acidity of the CH is largely varied by the internal rotation
of ethyl groups.
Correlation of the acidity and the band intensity
Increase of the acidity of CH ⇔ Increase of the intensity of the CH band
J. Mol. Struc. 113, 323 (1984). J. Mol. Struc. 224 363 (1990). J. Mol. Struc. 521, 1 (2000) など
Approximation
str
I CH

str
I CH
qH
CH
2
0.25
rCH
 qH  qnH
rCH
CH 強度
: Absorption Intensity
CH
: Atomic charge of H
rCH
 rCH
2
: dipole moment (CH)
:CH bond length
J. Mol. Struc. 521, 1 (2000)
The structure dependency of energy, acidity and CH bond length
Acidity (kcal/mol)
Frequency (cm-1)
RC-H (Angstrom)
Energy (kcal/mol)
Internal rotation angle of
the ethyl group/degree
Internal rotation angle of
the ethyl group/degree
The energy, the acidity (the dissociation energy of H) and the CH
bond length change with the internal rotation angle of the ethyl
group.
Internal rotation dependence of methyl CH frequency
Frequency (cm-1)
Energy (Kcal/mol)
Internal rotation angle of
the methyl group/ degree
The CH stretch frequency also depends on the internal rotation
angle of the methyl group, but the change is very small.
Infrared spectra of DEE+-Ar and DES+-Ar
Difference of degree of
hyperconjugation
Cations are generated
@118 nm
DEE+-Ar
DES+-Ar
2400
2600
2800
3000
3200
wavenumber /cm-1
3400
3600
3800
The CH band of DES+ appears at higher frequency region than that
of DEE+ because the degree of hyperconjugation of DES+ is small.
Infrared spectra of DEE and DEE+
nCH :2800~3000 cm-1
DEE (neutral)
Intense band
appears!
nCH:2920 cm-1
DEE+ (cation)
generated by 118nm photoionization
Wavenumber / cm-1
In the infrared spectra of DEE+, the broad intense band appears at
2700 cm-1.
・The band at 2700 cm-1 is anormalously intense and broad.
・The ordinal CH stretch frequency region is 2800 ~ 3000cm-1.
Comparison of hyperconjugation of DEE+ and DES+
The hyperconjugation of DES+ is smaller than that of DEE+.
The hyperconjugation of DES+ is small because the energy difference
between the s orbital and the nonbonding orbital is large
Previous study
Trimethylamine dimer cation structure
The stable H+-transferred structure is formed.
CH of TMA is also acidic.
(trimethylamine)2+