Conformational stability from variable temperature FT-IR spectra of xenon solutions and ab initio calculations of trans 3-pentenenitrile and 3-methyl-3-butene nitrile

Article abstract of DOI:10.1016/S0022-2860(00)00910-8

The infrared (3500-40 cm^-1) spectra of gaseous, liquid and solid 3-methyl-3-butene nitrile, CH₂C(CH₃)CH₂CN, and trans 3-pentenenitrile, CH₃CHCHCH₂CN, have been recorded. Both the cis and ga

Full text of DOI:10.1016/S0022-2860(00)00910-8

Journal of Molecular Structure 563±564 ꢀ2001) 551±554
www.elsevier.nl/locate/molstruc
Conformational stability from variable temperature FT-IR spectra
of xenon solutions and ab initio calculations of trans
3
-pentenenitrile and 3-methyl-3-butene nitrile
*
G.A. Guirgis,S. Shen,B.R. Drew,J.R. Durig
Department of Chemistry, University of Missouri-Kansas City, 5100 Rockhill Road, Kansas City, MO 64110-2499, USA
Received 27 September 2000; accepted 23 October 2000
Abstract
The infrared ꢀ3500±40 cm ) spectra of gaseous,liquid and solid 3-methyl-3-butene nitrile,CH CꢀCH )CH CN,and trans 3-
2
1
2
3
2
3 2
pentenenitrile,CH CHCHCH CN,have been recorded. Both the cis and gauche conformers have been identi®ed for both
conformers in the ¯uid phases but only the cis form remains in the solid. Variable temperature ꢀ255 to 21008C) studies of the
infrared spectra of the samples dissolved in liquid xenon have been carried out. From these data,the enthalpy difference has
2
1
been determined to be 174 ^ 20 cm ꢀ2:08 ^ 0:24 kJ=mol† for 3-methyl-3-butene nitrile with the cis conformer the more
21
stable rotamer and 187 ^ 20 cm ꢀ2:23 ^ 0:24 kJ=mol† for trans 3-pentenenitrile,again with the cis rotamer the more stable
form. Complete equilibrium geometries have been obtained for both rotamers for both molecules by ab initio calculations
employing the 6-31Gꢀd),6-311Gꢀd ,p ),6-311 1 Gꢀd,p) and 6-311 1 Gꢀ2d,2p) basis sets at the levels of restricted Hartree±Fock
and/or Moller±Plesset to second order. Only the MP2/6-311 1 Gꢀ2d,2p) calculation gives the correct conformer stability for 3-
methyl-3-butene nitrile. q 2001 Elsevier Science B.V. All rights reserved.
Keywords: Conformational stability; Infrared and Raman spectra; 3-Methyl-3-butene nitrile; trans 3-Pentenenitrile
1
. Introduction
From these data it was concluded that the enthalpy
21
value was 336 ^ 61 cm ꢀ4:02 ^ 0:73 kJ=mol† with
the cis conformer the more stable rotamer. This
enthalpy value should be near that for the gas [3±7].
In order to determine the effect of substitution of a
methyl group on the ethylene moiety of 3-buteneni-
trile we have investigated the conformational stability
of 3-methyl-3-butene nitrile,CH CꢀCH )CH CN,and
We previously investigated the conformational
equilibria exhibited by 3-butenenitrile [1,2] ꢀallyl
cyanide) ꢀCH yCHCH CN),and determined that the
2
2
cis ꢀcyano group is eclipsing the double bond)
conformer is the more stable rotamer in the liquid
phase and the only form remaining in the solid.
From the more recent study [2],a variable tempera-
ture investigation of the infrared spectrum of the
sample dissolved in liquid xenon was carried out.
2
3
2
trans 3-pentenenitrile,CH CHCHCH CN,utilizing
3
2
variable temperature infrared spectra of xenon solu-
tions of these two molecules. In order to assign the
individual bands to the correct conformers we have
carried out MP2/6-31Gꢀd) ab initio calculations
utilizing scaling factors of 0.88 for the carbon±
hydrogen stretches,0.90 for the carbon±hydrogen
*
Corresponding author. Tel.: 11-816-235-1136; fax: 11-816-
35-5502.
E-mail address: durigj@umkc.edu ꢀJ.R. Durig).
2
0022-2860/01/$ - see front matter q 2001 Elsevier Science B.V. All rights reserved.
PII: S0022-2860ꢀ00)00910-8

552
G.A. Guirgis et al. / Journal of Molecular Structure 563±564 ꢀ2001) 551±554
Table 1
Temperature and intensity ratios from the conformational study of
with sodium cyanide at 150±1608C for 1 h. A sample
of trans 3-pentenenitrile was prepared by the reaction
of neat crotyl chloride with sodium cyanide at 150±
1608C. The samples were collected under vacuum and
then puri®ed with a low-temperature,low-pressure
fractionation column.
trans 3-pentenenitrile and 3-methyl-3 butene nitrile
T ꢀ8C)
3 2 2 3 2
1000/T ꢀK) CH CHCHCH CN CH CꢀCH )CH CN
2lnꢀI883/I839
)
2lnꢀI551/I570)
2
2
2
2
2
2
2
2
2
2
55
60
65
70
75
80
85
90
95
100
4.58
4.69
4.80
4.92
5.05
5.18
5.31
5.46
5.61
5.78
1.281
1.281
1.337
1.369
1.392
1.431
1.456
1.506
1.545
1.591
187^20
0.115
0.110
The mid-infrared spectra of the samples dissolved
in xenon as a function of temperature were recorded
on a Bruker model IFS 66 Fourier transform inter-
ferometer equipped with a Globar source,a Ge/KBr
beamsplitter and a DTGS detector. The spectra were
recorded at variable temperatures ranging from 255
to 21008C. All the spectra were collected with 100
0.068
0.033
0.007
20.039
20.063
20.088
20.142
20.164
174^20
2
1
scans at a resolution of 1.0 cm . The temperature
dependent spectral data for the conformer bands are
listed in Table 1.
2
1
DH ꢀcm
)
bends and skeletal stretches,and 1.0 for the skeletal
bends and asymmetric torsion. Additionally,complete
equilibrium geometries have been obtained for both
rotamers for both molecules employing a variety of
basis sets with full electron correlation by the pertur-
bation method [8] to second order in order to predict
the conformational stabilities.
3. Results and discussion
The LCAO-MO-SCF restricted Hartree±Fock
ꢀRHF) calculations were performed with the
Gaussian-98 program [9] using Gaussian-type basis
functions. The energy minima with respect to nuclear
coordinates were obtained by the simultaneous relaxa-
tion of all of the geometric parameters consistent with
the symmetry restrictions using the gradient method
of Pulay [10]. The structural optimization for both the
cis and the gauche conformers were carried out with
initial parameters taken from those of 3-butenenitrile
2
. Experimental
A sample of 3-methyl-3-butene nitrile was prepared
by the reaction of neat 3-chloro-2-methyl propene
[2]. The 6-31Gꢀd),6-311Gꢀd p, ),6-311 1 Gꢀd,p) and
6-311 1 Gꢀ2d,2p) basis sets were employed at the
level of RHF and/or Moller±Plesset ꢀMP2) to second
order with full electron correlation. For the trans 3-
pentenenitrile molecule,all of the calculations at the
MP2 level predict the cis conformer to be the more
stable rotamer. However,the value is rather small
until the 6-311 1 Gꢀ2d,2p) basis set is utilized
21
which gives an energy difference of 182 cm
ꢀ
ꢀ
2.18 kJ/mol) ꢀTable 2). The infrared spectrum
2
1
400±900 cm ) of the sample dissolved in liquid
xenon is shown in Fig. 1 and the peaks at 883,752,
2
56 and 449 cm are predicted as bands due to the
1
5
gauche conformer at 877,733,538 and 435 cm
2
1
,
respectively. All of these bands disappear from the
spectrum of the annealed solid. From these data it is
concluded that the cis conformer is the rotamer that
remains in the solid state. Also it should be noted that
21
Fig. 1. Infrared spectrum ꢀ900±400 cm ) of trans 3-pentenenitrile
in liquid xenon.

G.A. Guirgis et al. / Journal of Molecular Structure 563±564 ꢀ2001) 551±554
553
Table 2
Predicted total energies ꢀHartree) and energy differences ꢀcm ) for the two stable conformers of trans 3-pentenenitrile and 3-methyl-3-butene
2
1
nitrile
Method/basis set
trans 3-pentenenitrile
3-methyl-3-butene nitrile
2
1
21
Energy ꢀgauche) Energy ꢀcm )
Energy ꢀcis)
Energy ꢀgauche) Energy ꢀcm
)
Energy ꢀcis)
RHF/6-31Gꢀd,p)
2247.842160
2248.661536
±
2247.842440
2248.661200
±
61 ꢀgauche)
74 ꢀcis)
±
2247.841582
2248.662702
2248.886790
2248.893842
2248.962170
2247.841807
2248.662998
2248.887240
2248.894667
2248.962050
49 ꢀgauche)
65 ꢀgauche)
99 ꢀgauche)
181 ꢀgauche)
226 ꢀcis)
MP2ꢀfull)/6-31Gꢀd,p)
MP2ꢀfull)/6-311Gꢀd,p)
MP2ꢀfull)/6-3111Gꢀd,p)
MP2ꢀfull)/6-3111Gꢀ2d,2p)
2248.892294
2248.959332
2248.892225
2248.959103
15 ꢀcis)
182 ꢀcis)
the bands assigned as fundamentals of the gauche
conformer are less than one-half the intensity of the
corresponding bands for the cis form. For example,
band. Utilizing the van't Hoff equation, 2ln K ˆ
ꢀDH=RT† 2 ꢀDS=R†; where K is the intensity ratio
I
gauche cis
dependence of 2ln K versus 1/T. This conformer
/I ,we obtained DH from the slope of the
2
1
the 427 cm band which is assigned as the CyC±C
21
21
bend has twice the intensity of the 449 cm band for
the corresponding mode of the gauche conformer.
Since there are two equivalent gauche conformers
and the predicted intensities ꢀ0.4 km/mol) in the
infrared spectra are nearly the same,these relative
intensities are a strong indication that the cis form is
the more stable conformer.
pair gave a DH value of 187 ^ 20 cm
0:24 kJ=mol† with the cis conformer being the more
ꢀ2:24 ^
stable form.
A portion of the infrared spectrum ꢀ1100±
500 cm ) of 3-methyl-3-butene nitrile is shown in
Fig. 2 and the bands observed at 570,719,805,977
and 1025 cm are assigned as fundamentals of the
gauche conformer based on the predicted wave-
numbers from the ab initio MP2/6-31Gꢀd) calculation
,respectively.
These predictions are remarkably good and in every
21
21
In order to determine the enthalpy difference, DH,
between the conformers of trans 3-pentenenitrile,the
relative intensities of the 883 ꢀgauche) and 839 ꢀcis)
conformer doublet were measured as a function of
temperature from 255 to 21008C. The results are
summarized in Table 1 and show that the intensities
of the cis band increases,as the temperature is
decreased compared to the intensities of the gauche
21
of 572,718,815,970 and 1026 cm
case the corresponding cis mode is predicted with the
correct relative value,i.e. the CH rock is predicted at
3
21
21
1014 cm for the cis mode ꢀobserved at 1014 cm )
whereas the CH rock of the gauche conformer is
3
2
1
predicted at 1026 cm . None of these bands that
are assigned as gauche modes are present in the
solid which means the cis conformer is the more
stable form in this phase.
The relative intensity of many of the gauche bands
are stronger than the corresponding cis peaks but for
most of these cases the predicted relative intensities
are considerably larger. For example,the CH rock at
3
2
1
1
014 cm for the cis form has a predicted intensity of
.6 km/mol whereas the corresponding gauche mode
1
has a predicted intensity of 8.6 km/mol. Similar differ-
ences are also predicted for the CyCC deformations
2
21
at 551 ꢀcis) cm with a predicted intensity of 0.9 km/
2
1
mol whereas the gauche mode at 570 cm has a
predicted intensity of 2.8 km/mol. Therefore,the rela-
tive intensities of these bands cannot be conveniently
used to predict the more stable conformer in the gas
2
1
Fig. 2. Infrared spectrum ꢀ1100±500 cm ) of 3-methyl-3-butene
nitrile in liquid xenon.

554
G.A. Guirgis et al. / Journal of Molecular Structure 563±564 ꢀ2001) 551±554
phase ꢀxenon solution) without carrying out a
temperature dependant study of the infrared spectrum.
In order to determine the enthalpy difference, DH,
between the conformers of 3-methyl-3-butene nitrile,
the relative intensities of the pair of bands at 551 ꢀcis)
and 570 ꢀgauche) cm were measured as a function
of temperature from 255 to 21008C. The results are
summarized in Table 1 and show that the intensities of
the cis band increases,as the temperature is decreased
compared to the intensities of the gauche band.
Utilizing the van't Hoff equation,this conformer
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21
[
6] B.J. van der Veken,F.R. DeMunck,J. Chem. Phys. 97 ꢀ1992)
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3
[
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[
7] M.O. Bulanin,J. Mol. Struct. 19 ꢀ1973) 59.
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Robb,J.R. Cheeseman,V.G. Zakrzewski,J.A. Montgomery,
R.E. Stratmann,J.C. Burant,S. Dapprich,J.M. Millam,A.D.
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Pittsburgh,PA,1998.
2
1
pair gives a DH value of 174 ^ 20 cm
0
ꢀ2:08 ^
:24 kJ=mol† with the cis conformer the more stable
form. This value should be a reasonable estimate for
the enthalpy difference in the gas phase as discussed
elsewhere [3±7]. Since both of these molecules have
an ethalpy difference considerably smaller than the
2
1
value of 336 ^ 61 cm reported [2] for 3-buteneni-
trile,it brings into question the correctness of this
value. Therefore,the enthalpy value for 3-buteneni-
trile should be determined again.
[
10] P. Pulay,Mol. Phys. 17 ꢀ1969) 197.
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