ARTICLE pubs.acs.org/JPCA
Conformational Stability from Variable Temperature Infrared Spectra of Xenon Solutions, r0 Structural Parameters, and Ab Initio Calculations of Cyclopropylisocyanate James R. Durig,*,‡ Sarah Xiaohua Zhou,†,‡ Gamil A. Guirgis,§ and Charles J. Wurrey‡ ‡
Department of Chemistry, University of Missouri-Kansas City, Kansas City, Missouri 64110, United States Department of Chemistry and Biochemistry, College of Charleston, Charleston, South Carolina 29424, United States
§
bS Supporting Information ABSTRACT: Infrared spectra (4000 to 400 cm-1) of the gas and variable temperature xenon solutions, and the Raman spectrum of the liquid have been recorded for cyclopropylisocyanate. The enthalpy difference has been determined to be 77 ( 8 cm-1 (0.92 ( 0.10 kJ/mol) with the trans form more stable than the cis conformer with 59 ( 2% present at ambient temperature. By utilizing three rotational constants for each conformer, combined with structural parameters predicted from MP2(full)/6-311þG(d, p) calculations, the adjusted r0 parameters have been obtained. Heavy atom structural parameters for the trans [cis] conformers are the following: distances (Å) (C-C2,3) = 1.509(3) [1.509(3)], (C2-C3) = 1.523(3) [1.521(3)], (C-N) = 1.412(3) [1.411(3)], (NdC) =1.214(3) [1.212(3)], (CdO) = 1.163(3) [1.164(3)]; angles () — CCN = 116.7(5) [120.1(5)], — CNC = 136.3(5) [137.6(5)]. The centrifugal distortion constants have been predicted from ab initio and DFT calculations and are compared to the experimentally determined values.
’ INTRODUCTION Organoisocyanates and organoisothiocyanates provide interesting challenges to structural scientists for the experimental determination of their conformational stabilities and structural parameters because of the large CNC(X) angle, which gives rise to many of the molecules having low-frequency, large amplitude bending vibrations that may be very anharmonic. Additionally, the barrier to internal rotation about the bond to the CNdC(dX) group may be relatively small, which can result in nearly free internal rotation of the NCX moiety. Also, because of the very low frequency of the bending mode there may be a very large number of excited vibrational states populated even at dry ice temperature, which is the temperature usually used to record the normal microwave rotational spectra of such molecules. It is also difficult to obtain good structural parameters for many of these molecules from electron diffraction studies. The nearly free rotation of the NCX moiety for more than 50% of the molecules at ambient temperature, along with a large number of anharmonic bending vibrations of very low frequency in many excited states, result in a large number of undetermined parameters, making it difficult to determine conformational stabilities. Further confusion about the structure of several of these molecules has arisen from ab initio predictions of the structures and conformational stabilities of organo-NCX compounds from calculations with relatively small basis sets which often do not agree with the most stable conformer or the stable conformers determined experimentally. Also, in some cases the potential r 2011 American Chemical Society
wells are so shallow that they cannot accommodate a vibrational state, hence, the microwave determined rotational constants will be different from the conformation which has the re minimum. Therefore, we have been reinvestigating the conformational structural stabilities of a number of these molecules, particularly by recording variable temperature vibrational spectra of the gas or of liquid rare gas solutions. Additionally, we have obtained the heavy atom structural parameters as well as ab initio predictions of the conformational stabilities by utilizing much larger basis sets than those that were previously used and which predicted the most stable conformer incorrectly. We initiated these recent studies by investigations of methylisothiocyanate (CH3NCS)1 and methylisocyanate (CH3NCO),2 where free internal rotation was observed for the methyl rotors for both molecules and the r0 structural parameters were determined. These studies have been followed with spectroscopic and theoretical studies of ethylisothiocyanate (CH3CH2NCS)3 and ethylisocyanate (CH3CH2NCO).4 For the thio- compound, it was concluded from the microwave spectra5 that the most stable conformer was the trans form, whereas from ab initio calculations6 at the MP2 level with various basis sets up to TZVP, it was concluded that the gauche form was the most stable form. However, from a more recent spectroscopic and theoretical study7 using larger basis sets it was conclusively shown that the most stable form was the cis conformer. A Received: September 17, 2010 Revised: January 13, 2011 Published: March 02, 2011 2297
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The Journal of Physical Chemistry A similar study of ethylisocyanate with large basis sets predicted the depth of the cis well to range from 11 cm-1 to a high value of 31 cm-1 and with the largest basis set (425 for the MP2(full)ccPVQZ calculation) predicting a value of 19 cm-1. This is a strong indication that the gauche wells could not contain a bound vibrational energy level. Also, the spectroscopic data was only consistent with the most stable conformer being the cis form. As a continuation of these studies of pseudohalogen organic compounds we have again investigated the conformational stability and determined the structural parameters of cyclopropylisocyanate. In an earlier microwave, infrared and Raman spectral study8 it was concluded that there were two conformers present at ambient temperature, which were the most stable trans form and the cis form (Figure 1). However, from a later theoretical study,9 the authors claimed that the trans and cis forms previously reported are actually two gauche forms with dihedral [CNC(1)C(2)] angles of 144.9 and 324.3 based on MP2/631G(d) calculations. There was little information on how the calculations were performed and a figure in the article implied that this dihedral angle should be 180 for the cis form. There was also a later microwave study.10 Since in the earlier study,8 the value of the A rotational constants for both conformers had significant uncertainties, a much larger number of transitions were assigned and measured for both conformers. For the cis conformer, it was not possible to fit the five usually determined centrifugal distortion constants by the standard equations, which lead the authors to question the conformation for the form assigned as the cis conformer. Thus, there is still a question concerning the stable conformers for cyclopropylisocyanate. Additionally, in the earlier study8 the enthalpy difference was determined for the liquid and only estimates were made for the r(C-N), — CNC, and — CCN parameters. Therefore, we have carried out a variable temperature infrared study of a xenon solution of cyclopropylisocyanate which should give an enthalpy value near that of the gas. Also, with three well-determined rotational constants it should be possible to obtain reliable structural parameters, and with ab initio calculations using much larger basis sets the predicted conformational stability is expected to be more reliable. The results of this spectroscopic and theoretical study are reported herein.
’ EXPERIMENT AND THEORETICAL CALCULATIONS The sample of cyclopropylisocyanate was prepared via a twostep procedure according to the method of Kricheldorf and Regel.11,12 In the first step, an intermediate product of diphenyl diazido silane was formed from dichloro diphenyl silane and sodium azide. The final product is achieved through the Curtius rearrangement of the intermediate formed from reaction with
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cyclopropyl carbonyl chloride. The sample was purified by trapto-trap distillation and the final purification was obtained by using a low-temperature, low-pressure sublimation column. The sample identity was verified by NMR and infrared spectroscopic data. The mid-infrared spectra from 3500 to 400 cm-1 of the gas shown in Figure 2A was recorded on a Perkin-Elmer model 2000 Fourier transform spectrometer equipped with a nichrome wire source, Ge/CsI beamsplitter and DTGS detector. The spectrum of the gas was obtained with the samples contained in 12 cm cells equipped with CsI windows. Atmospheric water vapor was removed from the spectrometer chamber by purging with dry nitrogen. Interferograms obtained after 128 scans for the gas sample and the background reference were transformed by using a boxcar apodization function with theoretical resolutions of 0.5 cm-1 for the gaseous sample. The mid-infrared spectra of cyclopropylisocyanate dissolved in liquefied xenon (Figure 2B) were recorded on a Bruker model IFS-66 Fourier interferometer equipped with a Globar source, Ge/KBr beamsplitter, and DTGS detector. The interferograms were recorded at variable temperatures ranging from -55 to 100 C with 100 scans and transformed by a Blackman-Harris apodization function with a theoretical resolution of 1.0 cm-1. The temperature studies in liquefied xenon were carried out in a specially designed cryostat cell, which is composed of a copper cell with a 4 cm path length and wedged silicon windows sealed to the cell with indium gaskets. The temperature was monitored by two platinum thermoresistors and the cell was cooled by the vapors from boiling liquid nitrogen. All of the observed fundamental
Figure 2. Infrared spectra of cyclopropylisocyanate: (A) gas; (B) xenon solution at -100 C. The asterisk indicates the spectral region at ∼600 cm-1 has nearly zero energy due to the absorption from the silicon windows.
Figure 1. Possible conformations of cyclopropylisocyanate, δ indicates the dihedral angle CdN-C-H4. 2298
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modes for the trans and cis conformers in the infrared and Raman spectra are listed in Tables 1 and 2, respectively. The Raman spectra of the liquid (Figure 3) was recorded on a Spex model 1403 spectrophotometer equipped with a SpectraPhysics model 2017 argon ion laser operating on the 514.5 nm line. The laser power used was 0.5 W with a spectral bandpass of 3 cm-1. The spectrum of the liquid was recorded with the sample sealed in a Pyrex glass capillary. The Raman spectrum of the gas (Figure 4A) at ambient vapor pressure was recorded by using the standard Cary multipass cell with 1-W laser power at the sample. The LCAO-MO-SCF calculations were performed with the Gaussian 03 program13 by using Gaussian-type basis functions. The energy minima with respect to nuclear coordinates were obtained by simultaneous relaxation of all geometric parameters consistent with symmetry restrictions using the gradient method of Pulay.14 A number of basis sets starting from 6-31G(d), and increasing to 6-311þG(3df,3pd), were employed at the level of Møller-Plesset perturbation theory15 to the second order (MP2), as well as hybrid density functional theory by the B3LYP method, to obtain energy differences (Table 3) among the three most likely conformers of cyclopropylisocyanate. At all levels of calculation carried out, with and without diffuse functions, predicted conformational stabilities varied extensively with the size of the basis set.
To aid in making the vibrational assignment (Tables 1 and 2), we have carried out a normal coordinate analysis by utilizing the force fields obtained from the Gaussian 03 program at the MP2(full)/6-31G(d) level. The internal coordinates used to calculate the G and B matrices for cyclopropylisocyanate are listed along with the structural parameters in Table 5. By using the B matrix, the force field in Cartesian coordinates was converted to a force field in internal coordinates.16 Subsequently, scaling factors of 0.88 for the CH stretches and 0.90 for all other modes were used, along with the geometric average of scaling factors for interaction force constants, to obtain the fixed scaled force fields and the resultant wavenumbers (Table 1). A set of symmetry coordinates (Table 1S) was used to determine the corresponding potential energy distributions (P.E.D.s). The observed and calculated wavenumbers of cyclopropylisocyanate along with the calculated infrared intensities, Raman activities, depolarization ratios, and P.E.D.s are given in Tables 1 and 2. To identify the fundamental vibrations for possible conformers of cyclopropylisocyanate, we have simulated the Raman spectrum (Figures 3 and 4) from the scaled ab initio MP2(full)/ 6-31G(d) results. The evaluation of Raman activity by using the analytical gradient method has been developed.17,18 The activity Sj can be expressed as: Sj = gj (45R2j þ 7β2j ), where gj is the degeneracy of the vibrational mode j, Rj is the derivative of the
Table 1. Calculated and Observed Vibrational Frequencies (cm-1) of Cyclopropylisocyanate for the trans Conformer infrared vib. No. 0
A
A00
approx. description
MP2/6-31G(d) MP2 scaleda IR int.b Raman act.c dp ratio gas
Xe
Raman
contour
gasd liquid
PEDe
A
C
ν1
CH2 antisym stretch
3320
3114
5.2
34.9
0.73
3109 3097 3107 3095
99S1
43
57
ν2
CH stretch
3229
3029
11.0
125.6
0.12
3036 3034 3034 3025
89S2, 10S3
35
65
ν3
CH2 symmetric stretch
3218
3019
0.9
98.3
0.06
88S3, 11S2
59
41
ν4
NCO antisym stretch
2410
2286
938.4
3.7
0.16
2281 2277 2289 2281
98S4
99
0
ν5
CH2 deformation
1568
1488
32.8
29.2
0.34
1480 1476 1482 1472
62S5, 18S8
100 0
ν6
NCO symmetric stretch
1515
1437
18.1
56.1
0.23
1446 1448 1449 1445
39S6, 26S5, 25S13
80
20
ν7
CH bend
1402
1330
53.0
8.3
0.74
1344 1342
99
1
ν8 ν9
ring breathing CH2 twist
1272 1176
1207 1115
0.3 4.2
16.9 4.1
0.15 0.62
1205 1201 1201 1200 1116 1115
65S8 38S9, 21S12, 16S7
96 30
4 69
ν10
CH2 wag
1111
1054
11.6
0.3
0.75
1032 1026 1034
92S10
72
28
ν11
ring deformation
979
929
7.4
9.1
0.60
918
67S11, 13S13
10
90
ν12
CH2 rock
826
784
1.4
3.0
0.28
∼800 ∼800 797
67S12, 13S13
0
100
ν13
ringC-N stretch
761
724
7.9
11.7
0.42
733
734 730
23S13, 34S9, 16S11
2
98
ν14
NCO bend
616
614
23.1
2.0
0.65
630
635 632
82S14
52
48
ν15
CC-N bend
371
371
12.4
3.5
0.31
369
367 376
77S15
93
7
ν16 ν17
CNC bend CH2 antisym stretch
130 3309
130 3104
6.7 0.4
2.6 76.5
0.72 0.75
126d
100 0
3095
88S16 99S17
ν18
CH2 symmetric stretch
3215
3016
6.6
27.6
0.75
3021
100S18
ν19
CH2 deformation
1515
1437
3.2
10.0
0.75
1445
100S19
ν20
CH2 twist
1236
1173
0.2
7.8
0.75
1167
ν21
CH bend
1158
1099
1.0
1.9
0.75
1091
ν22
CH2 wag
1110
1053
3.6
0.2
0.75
1050 1045
ν23
ring deformation
990
939
11.4
11.7
0.75
936
∼930
932
64S23, 14S26, 12S24
ν24 ν25
CH2 rock NCO bend
854 543
810 543
8.7 15.6
5.2 0.7
0.75 0.75
811 566
810 566
819 570
32S24, 30S23, 20S21, 16S20 100S25
ν26
R-NCO bend
413
413
2.9
0.4
0.75
416
418
ν27
ringC-N bend
12
12
0.5
3.0
0.75
3018
915 735
∼1350 64S7, 20S6
920 913
1172
52S20, 42S24 69S21, 22S20 94S22
87S26, 10S24 100S27
a Force constant scaling factors: 0.88 for CH stretches; 0.9 for all other modes. b Infrared intensities in km/mol. c Raman activities in Å4/u. d Ref 8. e PEDs from MP2 scaled calculation: values less than 10% are omitted.
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Table 2. Calculated and Observed Vibrational Frequencies (cm-1) of Cyclopropylisocyanate for the cis Conformer infrared
0
A
A00
vib. No.
approx. description
MP2/6-31G(d) MP2 scaleda IR int.b Raman act.c dp ratio gas
Raman
contour
Xe gasd liquid
PEDe
A
B
ν1
CH2 antisymmetric stretch
3313
3108
7.3
32.1
0.50
3102 3097 3107 3095 99S1
89
11
ν2
CH stretch
3259
3057
9.7
100.2
0.26
3053 3047 3054 3053 98S2
59
41
ν3
CH2 symmetric stretch
3214
3015
0.9
135.4
0.06
99S3
57
43
ν4
NCO antisymmetric stretch
2406
2282
797.6
2.2
0.09
98S4
88
12
ν5
CH2 deformation
1564
1484
9.2
17.3
0.52
1465 1472 1465 1470 77S5, 15S8
23
77
ν6
NCO symmetric stretch
1515
1437
15.7
48.5
0.17
1446 1448
1445 53S6, 33S13
93
7
ν7
CH bend
1439
1366
9.8
7.4
0.53
79S7
97
3
ν8 ν9
ring breathing CH2 twist
1264 1154
1199 1095
3.4 27.3
16.0 9.9
0.19 0.75
1197 1198 1106 1101
42 97
58 3
ν10
CH2 wag
1107
1050
19.5
0.3
0.73
1048 1045
ν11
ring deformation
971
920
0.9
11.9
0.51
ν12
CH2 rock
822
780
8.7
1.6
0.24
ν13
ringC-N stretch
763
725
14.7
9.7
ν14
NCO bend
624
620
24.1
ν15
CC-N bend
424
424
ν16 ν17
CNC bend CH2 antisymmetric stretch
107 3302
ν18
CH2 symmetric stretch
ν19
CH2 deformation
ν20
CH2 twist
1235
1172
0.5
7.9
0.75
1175 1167
1172 45S20, 44S24, 10S21
ν21
CH bend
1169
1109
0.7
1.2
0.75
1112 1112
68S21, 27S20
ν22
CH2 wag
1108
1051
3.7
0.1
0.75
1031 1026
94S22
ν23
ring deformation
991
940
10.7
8.2
0.75
930 928
63S23, 14S26, 12S24
ν24 ν25
CH2 rock NCO bend
856 551
812 551
8.6 18.2
5.9 0.5
0.75 0.75
805 805 571 566
33S24, 31S23, 17S20, 16S21 100S25
ν26
R-NCO bend
415
315
2.4
0.6
0.75
408 405
88S26
ν27
ringC-N bend
46
46
1.1
2.4
0.75
∼40d
100S27
1200 63S8, 11S9 1105 36S9, 22S12, 14S7, 14S6 94S10
9
91
65S11, 17S13, 11S6
0
100
788 787 792 792
66S12, 12S13, 10S9
81
19
0.32
725 726
26S13, 26S9, 23S11
33
67
1.0
0.74
615 613 623
78S14, 11S16
31
69
15.6
1.4
0.49
72S15
64
36
107 3098
3.6 0.5
3.2 73.2
0.58 0.75
88S16, 11S15 3095 100S17
92
8
3210
3011
6.8
21.0
0.75
1520
1442
3.5
8.2
0.75
920 726
107
100S18 1448
1445 100S19
a Force constant scaling factors: 0.88 for CH stretches; 0.9 for all other modes. b Infrared intensities in km/mol. c Raman activities in Å4/u. d Ref 8. e PEDs from MP2 scaled calculation: values less than 10% are omitted.
isotropic polarizability, and βj is that of the anisotropic polarizability. The Raman scattering cross sections, ∂σj/∂Ω, which are proportional to Raman activities, can be calculated from the scattering activities as well as the predicted wavenumbers for each normal mode, by using the relationship:19,20 ∂σj/∂Ω = [(2π)4/45] [(ν0 - νj)4/(1 - exp(-hcνj/kT))] [h/(8π2cνj)] Sj, where ν0 is the excitation wavenumber, νj is the vibrational wavenumber of the jth normal mode, and Sj is the corresponding Raman scattering activity. To obtain the polarized Raman scattering cross sections, the polarizabilities are incorporated into Sj by multiplying Sj by (1 - Fj)/(1 þ Fj), where Fj is the depolarization ratio of the jth normal mode. The Raman scattering cross sections and calculated wavenumbers obtained from the scaled ab initio force fields were used together with a Lorentzian function to obtain the simulated Raman spectra. To further support the vibrational assignments, the infrared spectrum (Figure 5) was predicted by using scaled wavenumbers from MP2(full)/6-31G(d) results. Infrared intensities were calculated based on the dipole moment derivatives with respect to Cartesian coordinates. The derivatives were transformed into normal coordinate derivatives by (∂μu/∂Q i) = Σj (∂μu/∂Xj) Lij, where Qi is the ith normal coordinate, Xj is the jth Cartesian displacement coordinate, and Lij is the transformation matrix between the Cartesian displacement coordinates and the normal
coordinates. The infrared intensities were then calculated by [(Nπ)/(3c2)][(∂μx/∂Q i)2 þ (∂μy/∂Q i)2 þ (∂μz/∂Q i)2]. Vibrational Assignment. To determine the enthalpy difference between the two conformers it is necessary to provide vibrational assignments for the fundamentals of each conformer in the region where the conformer pairs will be chosen for the enthalpy determination. Because the lower frequency region will have the fewest number of possible overtones or combination bands, the assignments for the fundamentals in this region will be very important to make before selecting pairs of conformer bands. However, because of the large amplitude motion of the NCO and R-NCO bends many of the bands below 700 cm-1 are very broad in the xenon solution, which makes it difficult to confidently assign them to the correct conformer. However, in the region between 700 to 1400 cm-1 most of the fundamentals can be indentified for each conformer; but in several cases the similar vibrations for the two conformers are observed within a very few wavenumbers from each other as predicted from the ab initio calculations. To make the assignments, considerable reliance was placed on the predicted fundamental frequencies and intensities from the ab initio calculations as well as the infrared band contours in the gas. From these experimental data most of the fundamentals in the 700 to 1400 cm-1 could be assigned with confidence. 2300
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Table 3. Calculated Electronic Energiesa (hartree) and Energy Differencesb (cm-1) for the trans, cis, and the skew Conformers of Cyclopropylisocyanate number of basis functions
trans
cis
skewc
MP2(full)/6-31G(d)
100
0.531149
52
105
MP2(full)/cc-PVDZ
109
0.569719
84
135
MP2(full)/6-31þG(d)
124
0.548210
95
173
MP2(full)/6-31G(d,p) MP2(full)/6-31þG(d,p)
115 139
0.572075 0.588575
57 97
112 184
MP2(full)/TZVP
144
0.778199
14
110
MP2(full)/6-311G(d,p)
138
0.767042
36
90
MP2(full)/6-311þG(d,p)
162
0.776779
7
124
MP2(full)/aug-cc-PVDZ
183
0.623931
19
84
MP2(full)/6-311G(2d,2p)
183
0.839084
75
122
MP2(full)/6-311þG(2d,2p)
207
0.847659
54
121
MP2(full)/6-311G(2df,2pd) MP2(full)/6-311þG(2df,2pd)
250 274
0.943928 0.951234
39 16
97 96
method/basis set
Figure 3. Observed (room temperature) and predicted (MP2(full)/ 6-31G(d) at 25 C) Raman spectra of cyclopropylisocyanate: (A) liquid; (B) predicted spectrum of the mixture of cis and trans conformers with ΔH = 77 cm-1; (C) predicted spectrum of the pure cis conformer; (D) predicted spectrum of the pure trans conformer; (E) solid. Asterisk indicates that the intensity has been reduced by half.
MP2(full)/6-311G(3df,3pd)
295
0.968773
72
129
MP2(full)/aug-cc-PVTZ
391
1.960505
53
133
B3LYP/6-31G(d)
100
1.372666
203
237
B3LYP/6-31þG(d)
124
1.384722
250
284
B3LYP/6-311G(d,p)
138
1.453174
257
279
B3LYP/6-311þG(d,p)
162
1.458582
232
264
B3LYP/6-311G(2d,2p) B3LYP/6-311þG(2d,2p)
183 207
1.461373 1.466843
271 226
285 250
B3LYP/6-311G(2df,2pd)
250
1.472328
243
268
B3LYP/6-311þG(2df,2pd)
274
1.477358
209
241
B3LYP/6-311G(3df,3pd)
295
1.476357
256
285
B3LYP/6-311þG(3df,3pd)
319
1.480813
198
233
a Energy of trans conformer is given as -(E þ 284) hartree. b All energy differences are relative to the energy of the trans conformer. c Transition state.
Figure 4. Observed (room temperature) and predicted (MP2(full)/ 6-31G(d) at 25 C) Raman spectra of cyclopropylisocyanate: (A) gas; (B) predicted spectrum of the mixture of cis and trans conformers with ΔH = 77 cm-1; (C) predicted spectrum of the pure cis conformer; and (D) predicted spectrum of the pure trans conformer.
The assignments for the fundamentals for the trans form are shown in Table 1 and for the cis conformer in Table 2. For some of the fundamentals with B-type band contours it was very difficult to determine the band center, but for some cases the infrared spectrum of the xenon solution could aid significantly in the band center determination. Nevertheless, there is a broad
nondescript band centered at 1106 cm-1 where two fundamentals for each conformer are predicted. The assignments of the fundamentals are somewhat arbitrary because there is little band contour or intensity information to aid in the identification of the band centers. A similar problem arises in the region 1130 to 1220 cm-1, which makes it difficult to confidently assign the four fundamentals predicted in this range at 1207 (ν8, trans), 1173 (ν20, trans), 1199 (ν8, cis), and 1172 (ν20, cis). All except the 1199 cm-1 band are predicted to have very low intensities and the band contours in this region are very ill defined. Therefore, the infrared spectrum of the xenon solution and Raman spectrum of the liquid were needed to determine the band centers. Only two bands (1198 and 1167 cm-1) can be readily observed in the xenon solution while only one very strong band (1200 cm-1) is observed in the Raman spectrum of the liquid. The band in the xenon solution at 1198 cm-1 is assigned as the ν8 (cis) mode and the 1167 cm-1 bands is assigned as the ν20 (cis) mode. With these assignments, corresponding band centers were identified in the infrared spectrum of the gas at 1197 and 1175 cm-1, respectively. The corresponding Raman band at 1200 cm-1 in the spectrum of the liquid supports these assignments and the weaker Raman line at 1172 cm-1 is consistent with the lower frequency assignment. Enthalpy Difference. The initial ΔH determination8 was made from a single conformer pair from Raman data of the liquid. Unfortunately, the band for the cis conformer was due to stray light so its intensity did not decrease when the temperature 2301
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Figure 5. Observed and predicted (MP2(full)/6-31G(d)) infrared spectra of cyclopropylisocyanate: (A) xenon solution at -100 C; (B) predicted spectrum of the mixture of cis and trans conformers with ΔH = 77 cm-1; (C) predicted spectrum of the pure cis conformer; and (D) predicted spectrum of the pure trans conformer.
was lowered. Since it did not decrease, the ΔH was determined from essentially the increase of the fundamental due to the trans form when the temperature was lowered so the reported value is expected to be slightly smaller than the actual value. Therefore we have determined the enthalpy difference by a variable temperature study of the xenon solution. For the best results the lowest frequency fundamentals should be used for the conformer pairs since they have the lowest probability of having intensity contributions from overtones or combination bands. For the cis conformer the bands at 726 and 787 cm-1 have been confidently assigned to this form and for the trans form the fundamentals at 810, 915, 928, and 1026 cm-1 were utilized (Figure 2B). These bands were selected because they are well resolved and well separated, with relatively flat baselines, and they are without predicted underlying bands from the other conformer. From these fundamentals for each conformer, eight conformer pairs were selected for the conformational stability determination. The intensities of the infrared bands were measured as a function of temperature and their ratios were determined. By application of the van’t Hoff equation, -ln K = ΔH/(RT) - ΔS/R, ΔH was determined from a plot of -ln K versus 1/T (Figure 1S), where ΔH/R is the slope of the line and K is substituted with the appropriate intensity ratios, that is, Itrans/Icis. It is assumed that ΔH and ΔS are not functions of temperature in the temperature range studied. The resulting values with statistical uncertainties of one sigma for each set are listed in Table 4. The average of the eight values is 77 ( 3 cm-1 (0.92 ( 0.04 kJ/mol)) where the error limit is derived from the statistical standard deviation of one sigma of the measured intensity data, where the data are taken as a single set. Although the statistical uncertainties are relatively small, they do not take into account possible contribution from combination or overtone bands from other conformers contributing to the
ARTICLE
measured fundamental band intensities. The variations of ΔH values are undoubtedly due to these types of interferences but by taking eight conformer pairs it is expected that these effects are nearly canceled. However, the nature of the technique utilized for enthalpy determination is not expected to provide data better than 10%. Therefore, a more realistic ΔH value is 77 ( 8 cm-1 (0.92 ( 0.10 kJ/mol)). From this ΔH value, a realistic estimation of the abundance of the less stable cis conformer present at ambient temperature is 41 ( 2%. Structural Parameters. In the initial microwave study8 only the r(C1-N), — C2,3C1N, and — C-NdC were determined for both conformers from the three rotational constants for each form with the other parameters estimated from related molecules. The larger C-N distance and the smaller CCN angle of the trans conformer compared to the corresponding parameters of the cis form are consistent with the expected difference of these parameters. However, the two A rotational constants had rather large uncertainties which could have affected the values of the three determined parameters. Additionally, the values estimated from those obtained from other molecules may not be comparable to the values of similar parameters for cyclopropylisocyanate. Furthermore, it is suggested that by combining ab initio predicted values as constraints more reliable structural parameters could probably be obtained. We have found that good structural parameters for hydrocarbons and many substituted ones can be determined by adjusting the structural parameters obtained from the ab initio MP2/6-311þG(d,p) calculations to fit the rotational constants obtained from microwave experimental data by using a computer program “A&M” (ab initio and microwave) developed21 in our laboratory. To reduce the number of independent variables, the structural parameters are separated into sets according to their types. Bond lengths in the same set keep their relative ratio which results in only four heavy atom distances for cyclopropylisocyanate, bond angles and torsional angles in the same set keep their difference in degrees. This assumption is based on the fact that the errors from ab initio calculations are systematic. Additionally, we have also shown that the difference in predicted distances and angles from the ab initio calculations for different conformers of the same molecule can usually be used as one parameter, namely, the ab initio predicted difference except for some dihedral angles. However, for the trans and cis conformers, these dihedral angles are zero and 180 so they are not variable parameters. Thus, there are six heavy atom parameters to be adjusted, so it should be possible to obtain “adjusted r0” structural parameters for cyclopropylisocyanate by utilizing the six determined microwave rotational constants from the two conformers since the sets are the same for the two forms. We22 have also shown that ab initio MP2/6-311þG(d,p) calculations predict the r0 structural parameters for more than 50 carbon-hydrogen distances to better than 0.002 Å compared to the experimentally determined values from isolated CH stretching frequencies, which were compared to previously determined23 values from earlier microwave studies. Therefore, all of the carbon-hydrogen distances were taken from the MP2/ 6-311þG(d,p) predicted values for cyclopropylisocyanate. Therefore, a complete structure has been determined for each of the two conformers. The determined adjusted r0 parameters are listed in Table 5 and the final fit of the rotational constants is shown in Table 6. The differences are quite small with all six rotational constants fit to better than 1.0 MHz. The bond distances that changed the most during the fitting are r(C2-C3), r(C1-N), and r(CdO), 2302
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Table 4. Temperature and Intensity Ratios of the Conformer Bands of Cyclopropylisocyanate from the Infrared Spectra of the Liquid Xenon Solution T (C)
1/T ( 10-3 K-1)
I810/I726
I810/I787
I928/I726
I928/I787
I1026/I726
I1026/I787
I1342/I726
I1342/I787
liquid
-60.0
4.6915
2.6141
12.7273
1.9835
4.9618
8.0745
36.1111
5.3114
23.3764
xenon
-65.0 -70.0
4.8042 4.9225
2.6281 2.6721
13.1950 13.4683
2.0410 2.0661
4.7980 4.9505
8.0116 8.0674
37.5000 38.5366
5.3696 5.3331
23.8948 23.9598
-75.0
5.0467
2.6770
13.1596
2.0891
5.0292
8.2652
39.0986
5.4850
24.0773
-80.0
5.1773
2.7893
13.6409
2.0722
5.1135
8.5417
38.8182
5.5966
24.2691
-85.0
5.3149
2.8239
13.9234
2.0954
5.2184
8.7174
39.8649
5.6052
25.1982
-90.0
5.4600
2.8493
14.0000
2.1005
5.2857
8.6462
40.7333
5.6632
25.6448
-95.0
5.6132
2.9066
14.5783
2.1923
5.3826
8.8091
41.7391
5.7683
26.3534
-100.0
5.7753
2.9042
14.6962
2.2556
5.5291
8.8769
41.7722
5.8333
26.6806
77 ( 7
87 ( 9
66 ( 10
82 ( 9
73 ( 9
88 ( 9
63 ( 5
87 ( 7
ΔHa
a
Average value obtained by utilizing all data as a single set gives ΔH = 77 ( 3 cm-1 (0.92 ( 0.03 kJ/mol) with the trans conformer more stable.
Table 5. Structural Parameters (Å and degree), Rotational Constants (MHz), and Dipole Moments (debye) for the trans and cis Conformers, Respectively, of Cyclopropylisocyanate from Ab Initio Calculations (6-311þG(d,p) Basis Set) and Experimental Data trans parametera
int. coord.
MP2
B3LYP
r(C1-C2,3)
R1, R2
1.504
1.507
r(C2-C3)
R3
1.512
1.510
r(C1-N)
R4
1.425
r(NdC)
R5
r(CdO) r(C1-H4) r(C2,3-H8,10)
MWc
cis gauche-1b
gauche-2b
trans
adjusted r0d trans
cis
1.520
1.509(3)
1.509(3)
1.515
1.523(3)
1.521(3)
1.417(10)
1.407(14)
1.412(3)
1.411(3)
1.216
1.210
1.210
1.214(3)
1.212(3)
1.175
1.191
1.170
1.170
1.163(3)
1.164(3)
1.082 1.083
1.079 1.079
1.079 1.086
1.079 1.086
1.085(2) 1.083(2)
1.082(2) 1.084(2)
1.083
1.083
1.079
1.082
1.082
60.4
60.2
60.2
MP2
B3LYP
1.499
1.505
1.508
1.501
1.520
1.504
1.516
1.513
1.506
1.515
1.425
1.421
1.423
1.421
1.419
1.216
1.200
1.215
1.215
1.198
R6
1.179
1.174
1.190
1.180
r5 r1, r3
1.085 1.083
1.084 1.082
1.081 1.078/1.079
1.082 1.084
r(C2,3-H9,11)
r2, r4
1.083
1.083
1.079
— C2C1C3
π1
60.4
60.1
60.2
cis
1.083(2)
1.083(2)
60.6(5)
60.5(5)
— C2,3C1N
ε1, ε2
117.2
118.1
117.4
120.2
121.1
120.6
117.9(11)
120.3(24)
116.7(5)
120.1(5)
— C-NdC
ψ
134.3
139.0
137.4
135.3
142.1
137.9
136.9(38)
138.6(13)
136.3(5)
137.6(5)
172.7
172.6
172.6
172.2(5)
173.0(5)
115.4(5)
112.0(5)
117.9(5) 117.2(5)
117.7(5) 117.3(5)
— NdC = O
θ
172.9
174.1
172.7
172.9
174.2
— NC1H4
ζ
115.4
114.5
115.2
112.0
111.5
— C2,3C1H — C2,3C3,2H10,8
σ1, σ2 β3, β1
117.9 117.2
117.7 117.6
117.9 118.0/117.5
117.6 117.2
117.1 117.5
— C2,3C3,2H11,9
β4, β2
118.4
118.9
118.3/118.9
118.5
119.0
118.5(5)
118.4(5)
— C1C2,3H8,10
R1, R3
116.2
116.7
116.4/116.3
116.6
117.2
116.4/116.7
116.2(5)
116.7(5)
— C1C2,3H9,11
R2, R4
117.6
117.6
117.8/117.8
117.9
117.8
118.0/118.0
117.6(5)
117.8(5)
CdN-C1-C2
145.6
145.4
144.9
324.4
324.1
324.3
145.6(5)
324.4(5)
A
17016
17190
17218
9997
10706
10346
16691(14)
10161(13)
16941.7
10214.9
B
1769
1758
1753
2208
2085
2139
1784.303(3)
2186.797(10)
1783.4
2186.6
C |μa|
1706 2.997
1693 2.966
1691
2114 3.288
2029 3.217
2063
1716.133(3) 2.56(2)
2106.242(10) 2.720(4)
1717.0
2106.5
0.079
0.076
|μc|
1.061
0.867
|μtot|
3.179
3.090
|μb|
117.5
116.1
116.1
0.17(0.01) 0.71(3)
2.91
3.289
3.218
2.99
2.65(2)
2.726(1)
a
cis and trans refers to the orientation of the hydrogen relative H4. b Ref 9. Optimized geometry determined from MP2/6-31G(d). c Structural parameter from ref 8. d Rotational constants utilized for the fitting are from ref 10.
these adjusted r0 parameters differ by at least 0.010 Å from their predicted values for both of the conformers. The largest adjustment for the bond angle is associated with the C-NdC angle, where the adjusted r0 is at least 2 larger than the MP2(full)/ 6-311þG(d, p) predicted value. The predicted structural parameters from the MP2(full)6-311þG(d,p) calculations for both the trans and cis conformers
are listed in Table 5 along with those from B3LYP/6-311þG(d,p) calculations. These are compared to those previously reported for the two conformers indicated as gauche-1 and gauche-2, which were obtained9 from ab initio predictions from MP2/6-31G(d,p) calculations. By using the dihedral CdN-C-C2 angle as an indication of the conformation, the gauche-1 form reported differs by only 0.7 from this angle for the trans form and only 2303
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ARTICLE
Table 6. Comparison of Rotational Constants (MHz) Obtained from Modified Ab Initio MP2(full)/6-311þG(d,p) Predictions, Microwave Spectra, and the Adjusted Structural Parameters for Cyclopropylisocyanate conformer
rotational constant
MP2(full)/6-311þG(d,p)
microwavea
adjusted r0
|Δ|
trans
A
17018.0
16941.889(8)
16941.4
0.5
B C
1769.3 1706.2
1784.3121(6) 1716.1278(6)
1783.4 1717.0
0.9 0.9
A
10006.4
10215(6)
10214.9
0.1
B
2206.2
2186.859(6)
2186.6
0.3
C
2113.2
2106.172(6)
2106.5
0.3
cis
a
Rotational constants obtained from a fourth order centrifugal distortion fit, ref 10.
0.1 difference for the assigned gauche-2 form for this angle for the cis conformer. Further clarification concerning the incorrect dihedral angle that was used for determining the gauche forms versus cis or trans will be provided in the discussion
’ DISCUSSION The vibrational assignments given earlier8 were primarily based on infrared band contours, Raman depolarization data, and group frequencies. For those presented in the current study there is significantly more information, which includes the predicted frequencies for the fundamentals, the infrared intensities, and Raman activities, with probably the most important data being the infrared spectra of variable temperature xenon solution from which the bands for the two different conformers could be identified. Therefore, a few of the previously assigned bands as fundamentals, such as the 1005 cm-1 band due to stray light, as well as the bands at 823 and 351 cm-1, have been reassigned. There are some differences because the band centers have different frequencies in some cases. A major problem arises due to the significant breadth of several of the fundamentals of the trans conformer, particularly those arising mainly from motions of the NCO group. Most of the fundamentals for the cis conformer are very broad, which is probably due to only two energy levels in the potential well for this conformer since transitions for only one excited state were observed in the microwave spectrum. For the trans conformer four excited states of the low frequency NCO bend (torsion) were obtained. Therefore many of the fundamentals for the cis conformer have bands with nondescript contours. The ab initio predicted frequencies for the fundamentals of the trans form with two scaling factors of 0.88 for the CH stretches, 0.90 for the CH bends, and heavy atom stretches and no scaling for the other modes agreed with the modes of A0 symmetry with an average frequency difference of 8.0 cm-1, which is 0.6% error. For the A00 modes, the average difference is 9.1 cm-1 (0.8%), which indicates the value of using the relatively small basis set for aiding the assignment. The descriptions for the vibrational motions usually have only one or two symmetry coordinate contributions with four modes having contributions from three symmetry coordinates and only one (ν24) with four contributors with the largest one contributing only 32%. This latter mode has nearly equal contributions from the CH2 rock and the ring deformation. Also, it should be noted that ν13 is described as the C-N stretch but only 23% of S23 contributes to the 730 cm-1 band assigned to this motion, which also has a significant contribution from the NCO symmetric stretch, and lesser amounts from ν11 and ν12. Nevertheless, for most of the vibrations the approximate description gives the major contribution to that vibration.
The enthalpy value obtained from the xenon solution is expected to be close to the value for the gas since only small interactions are expected to occur between the dissolved molecules and the surrounding noble gas atoms.24-27 Further, the “pseudo gas phase” spectrum shows only small frequency shifts compared to the spectrum of the gas. A significant advantage of this type of cryogenic spectroscopic study is that the conformer bands are better resolved in comparison with those in the spectrum of the gas. This is particularly important since most of the conformer bands for this molecule are expected to be observed within a few wavenumbers of each other. The different conformer bands which can be clearly identified in the spectral region between 1100 and 600 cm-1 of the xenon solution are shown in Figure 1 and the ones indicated are due to the individual conformers. The extremely small uncertainty in ΔH of (3 cm-1 (0.04 kJ/mol) is probably smaller than the technique justifies but it is the statistical value. Nevertheless, the determined value is certainly more accurate than the value that could have been obtained for the gas. There is support for the very small enthalpy value from the ab initio predicted values where the MP2(full)/aug-cc-PVTZ calculation with the largest basis set of 391 wave functions predicts an energy difference of only 20 cm-1. It is expected that this estimated value is the most reliable one because of the large basis set.28 Although large basis sets are necessary for the determination of the energy difference between conformers, much smaller ones can be utilized for the determination of quadrupole coupling constants and usually for the centrifugal distortion constant as well. Satisfactory quadrupole coupling constant values have been obtained for both the trans and cis conformer (Table 7) and centrifugal distortion constant values for the trans form. The structural parameters obtained from the earlier microwave study8 were obtained from the three rotational constants for each conformer. The parameters for the hydrogen atoms were fixed at the value reported from these parameters for cyclopropyl chloride29 and the structure for the NCO group from the reported parameters of vinyl isocyanate30,31 and isocyanic acid.32 The r(C1-C2) value was estimated from the planar moment33 and the C2-C3 distances of 1.515 Å for this molecule were taken from those of cyclopropylisothiocyanate (electron diffraction34). With these parameters fixed, the r(C-N), — CNC, and — CCN were obtained for both conformers by diagnostic least-squares adjustment.35 The previously assumed and determined parameters are listed in Table 5 and there are some significant differences from the parameters obtained in this study. For example, the assumed r(C1-C2,3) and r(CdO) seem entirely too long, whereas r(C1-H) is too short. Also, the determined difference of 0.010 Å between the two values of r(C1-N) is too 2304
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ARTICLE
Table 7. Rotational (MHz), Centrifugal Distortion (kHz), and Quadrupole Coupling Constants for the trans and cis Conformers of Cyclopropylisocyanate trans
cis
MP2/6-31G(d) MP2/6- 311þG(d,p) B3LYP/6-311þG(d,p)
a
a
MW
MWa
MP2/6- 31G(d) MP2/6- 311þG(d,p) B3LYP/6-311þG(d,p)
A
17221
17016
17191
16941.889(8)
10359
9997
10705
10215(6)
B
1753
1769
1758
1784.3121(6)
2137
2208
2085
2186.859(6)
C DJ
1691 0.115
1706 0.114
1693 0.160
1716.1278(6) 0.153(2)
2062 1.504
2114 1.448
2029 1.550
2106.172(6) 2.01(1)
DJK
23.670
47.713
2.319
52.28(3)
3.140
-5.393
7.270
29.5(3)
DK
-2.843
-22.405
19.965
-6(3)
57.49
51.83
72.69
n.d
D1
-0.002
-0.001
-0.002
-0.027(1)
-0.22
-0.23
-0.22
-0.31(2)
D2
-0.002
-0.011
0.017
0.07
0.03
0.07
χaa
2.9179
2.9715
2.9504
2.6306(26)
2.6501
2.8092
2.8517
2.5647(49)
χbb
-1.7909
-1.8545
-1.5362
-1.3839(31)
-0.7920
-0.8843
-1.2248
-1.0624(75)
χcc
-1.1270
-1.1170
-1.4142
-1.2467(31)
-1.8581
-1.9249
-1.6269
-1.5022(75)
Ref 10; n.d.: not determined.
large because the predicted ab initio value is only 0.001 Å! Although the ab initio predicted value for the distance for one of the conformers may differ significantly from the experimental value, approximately the same difference is expected for the other conformer, so the predicted difference is usually quite accurately predicted. However, the listed uncertainty for the r(C1-N) distance could be interpreted as being essentially the same distance. The other parameter where there is a significant difference between the estimated value and the determined distance in this study is r(CdO), which is significantly shorter from this study. However, the previously determined — C2,3C1N and — C-NdC values are in good agreement with the value obtained for the corresponding parameters in the current study. In the second microwave study,10 the scientists were mainly interested in the value of the centrifugal distortion constants. The experimentally determined values of the quadratic distortion constants for the trans form are listed in Table 7 along with the value obtained from the force constants obtained from the ab initio and the density functional theory calculations. Often the prediction from B3LYP calculations are in better agreement than those from the MP2 prediction but in this case MP2/6-311þ G(d,p) values were quite superior, particularly for DJK and DK. However, for the cis form, reasonable centrifugal distortion values could not be obtained even by using the fourth order fit. In fact, these investigators10 initially suggested in the introduction that the conformational behavior of cyclopropylisocyanate raised some doubts about the existence of the cis form. Also, the authors10 attempted to explain the failure of the centrifugal distortion analysis for this conformer by using the basic assumptions of the conventional distortion theory for a rigid rotor Hamiltonian. For this Hamiltonian, the centrifugal distortion effects are described by a harmonic potential function and the theory allows only the small displacements of the internal coordinates from the equilibrium values to be considered. They explain that “These conditions are obviously not fulfilled for “cis” cyclopropylisocyanate. Because of the rigidity of the cyclopropyl frame and of the NCO group, especially the torsion around the Cframe-N bond, allows for flexibility of the molecule. If we assume a very flat and anharmonic potential near the “cis” conformation with a low barrier to the trans conformation, the failure of the centrifugal distortion analysis is plausible.”10 Nevertheless, the problem was not the question of cis being the correct
conformer, it is due to the need for a potential function that has been described as like an “inverted champagne bottle.”36 This type of potential simply arises from the large amplitude bending motion of the C-NCO linkage near the top of the potential well. It would be of interest to use such a potential to assign high J-level values, particularly values larger than J = 2. It was noted10 earlier that for K-1 = 3 the transitions showed a positive deviation, whereas those with K- > 3 were characterized by negative deviation. The authors of the theoretical paper9 seemingly did not take into consideration the information from the microwave studies.8,10 The dipole moment components determined8 from the Stark effects clearly indicated that the trans form had a plane of symmetry with |μb| = 0 and similarly for the cis conformer with |μc| = 0. The optimization of the two possible gauche equilibria9 from the MP2/6-31G(d,p) calculation gives the trans and cis conformations unless the gauche conformers are fixed as the structures. Apparently, from the earlier theoretical studies9 the investigators really did obtain the trans and cis conformers. In fact, the authors9 even acknowledged the similarity of the two “gauche equilibrium structures” with the trans and cis forms of the previous study.8 For the actual gauche and skew forms, the dipole moments obtained would have all three components unequal to zero (Table 2S). A more appropriate choice of the dihedral angle used would be CdN-C-H4 instead of CdN-C-C2. When CdN-C-H4 is used as the dihedral angle, the equilibrium structure for the trans and cis conformers would have a plane of symmetry and it would also clearly show how the NCO group is oriented relative to the ring if the molecule is in the two “gauche equilibrium structures.” We have carried out the MP2(full)/6-31G(d,p) calculation utilized in the earlier theoretical study.9 In Figure 6, the potential functions of MP2(full)/6-31G(d,p) and MP2(full)/6-311G(2d,2p) are shown, and the smaller basis set gives a much broader and shallower potential well for the trans conformer than the MP2(full)/6-311G(2d,2p) calculation. The depth of the well for the cis form is approximately the same from both calculations. Due to the breadth of this potential function from the smaller basis set, it is possible that the method utilized would make it difficult to determine the minimum because only two points were used between 90 and 180. Nevertheless, the MP2(full)6-31G(d, p) predicted the minimum at the trans and cis position. 2305
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The Journal of Physical Chemistry A
Figure 6. Potential energy curve of cyclopropylisocyanate as a function of the dihedral angle CdN-C1-H4 obtained from MP2(full)/ 6-31G(d,p) calculations (dashed line) and MP2(full)/6-311G(2d,2p) calculations (solid line).
If a more appropriate dihedral angle was used for indicating the conformers, the MP2(full)6-31G(d,p) from the theoretical study9 would have given the same result as this current study.
’ ASSOCIATED CONTENT
bS
Supporting Information. Table S1, symmetry coordinates for cyclopropylisocyanate; Table 2S, a comparison of cyclopropylisocyanate structural parameters (Å and degree), rotational constants (MHz), and dipole moments (debye) for the trans, cis, gauche, skew, gauche-1, and gauche-2 conformers from MP2/6-31G(d,p) calculation; and Figure 1S, van’t Hoff plot of the eight infrared conformer bands utilized in the determination of enthalpy difference value for cyclopropylisocyanate dissolved in liquid xenon solution. This material is available free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION Corresponding Author
*Phone: 01 816-235-6038. Fax: 01 816-235-2290. E-mail: durigj@ umkc.edu. Author Contributions †
Taken in part from the dissertation of S. X. Zhou, which will be submitted in partial fulfillment for the Ph.D. degree.
’ ACKNOWLEDGMENT J.R.D. acknowledges the University of Kansas City Trustees for a Faculty award and S.X.Z. acknowledges the Graduate Assistance Fund of the UMKC Women’s Council (Linda Hood Talbott Award, Eleanor Brantley Schwartz Award, Samuel L. and Peni P. Colville Award) for partial financial support of this research. ’ REFERENCES (1) Zheng, C.; Guirgis, G. A.; Deeb, H.; Durig, J. R. On the Structural Parameters and Vibrational Spectra of CH3NCS, SiH3NCS and GeH3NCS. J. Mol. Struct. 2007, 829, 88–110. (2) Zhou, S. X.; Durig, J. R. The r0 Structural Parameters, Vibrational Spectra, Ab Initio Calculations and Barriers to Internal
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Rotation and Linearity of Methylisocyanate. J. Mol. Struct. 2009, 924, 111–119. (3) Durig, J. R.; Zheng, C. Conformational Stability, Structural Parameters and Vibrational Assignment from Variable Temperature Infrared Spectra of Krypton Solutions and Ab Initio Calculations of Ethylisothiocyanate. Spectrochim. Acta 2007, 68, 783–795. (4) Durig, J. R.; Zhou, S. X.; Zheng, C.; Durig, D. T. Conformational Stability, Structural Parameters and Vibrational Assignment from Variable Temperature Infrared Spectra of Xenon and Krypton Solutions and Ab Initio Calculations of Ethylisocyanate. J. Mol. Struct. 2010, 971, 23–32. (5) Sakaizumi, T.; Ohashi, O.; Yamaguchi, I. Microwave Spectrum of Ethyl Isothiocyanate. Bull. Chem. Soc. Jpn. 1976, 49, 948–953. (6) Palmer, M. H.; Nelson, A. D. The Structures of the Azido-, Isocyanato- and Isothiocyanato-Derivatives of Methane and Silane and their Derivatives. A Comparison of Ab Initio with Experimental Results. J. Mol. Struct. 2004, 689, 161–173. (7) Durig, J. R.; Zheng, C. Conformational Stability, Structural Parameters and Vibrational Assignment from Variable Temperature Infrared Spectra of Krypton Solutions and Ab Initio calculations of Ethylisothiocyanate. Spectrochim. Acta 2007, 68, 783–795. (8) Durig, J. R.; Berry, R. J.; Wurrey, C. J. Microwave, Infrared and Raman Spectra, Conformational Stability, Structure, Dipole Moment and Vibrational Assignment for Cyclopropylisocyanate. J. Am. Chem. Soc. 1988, 110, 718–726. (9) Nguyen, M. T.; Hajnal, M. R.; Vanquickenborne, L. Q.; Ha, T.-K.; Stohner, J. Structure and Conformation of Chlorosulfonylisocyanate and Cyclopropylisocyanate. J. Chem. Soc., Faraday Trans. 1993, 89, 2381–2384. (10) Heldmann, C.; Dreizler, H. Investigation of the Microwave Spectrum of Cyclopropyl Isocyanate. An Example for the Failure of Centrifugal Distortion Theory. Z. Naturforsch. 1990, 45, 1175–1184. :: (11) Kricheldorf, H. R.; Regel, W. Reaktionen mit Silylaziden, 4. Uber Cyclopropylisocyanate. Chem. Ber. 1973, 106, 3753–3764. (12) Kricheldorf, H. R.; Regel, W. Reaktionen mit Silylaziden, 5. Notiz €uber die Acylierung und Alkylierung von Trimethylsilylazid. Chem. Ber. 1973, 106, 3765–3768. (13) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Montgomery, Jr., J. A.; Vreven, T.; Kudin, K. N.; Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.; Barone, V.; Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson, G. A.; Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Klene, M.; Li, X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.; Bakken, V.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Ayala, P. Y.; Morokuma, K.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Zakrzewski, V. G.; Dapprich, S.; Daniels, A. D.; Strain, M. C.; Farkas, O.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Ortiz, J. V.; Cui, Q.; Baboul, A. G.; Clifford, S.; Cioslowski, J.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara, A.; Challacombe, M.; Gill, P. M. W.; Johnson, B.; Chen, W.; Wong, M. W.; Gonzalez, C.; Pople, J. A. Gaussian 03, Revision E.01, Gaussian, Inc., Wallingford, CT, 2004. (14) Pulay, P. Ab Initio Calculation of Force Constants and Equilibrium Geometries. I. Theory. Mol. Phys. 1969, 17, 197–204. (15) Møller, C.; Plesset, M. S. Note on an Approximation Treatment for Many-Electron Systems. Phys. Rev. 1934, 46, 618–622. (16) Guirgis, G. A; Zhu, X.; Yu, Z.; Durig, J. R. Raman and Infrared Spectra, Conformational Stability, Normal Coordinate Analysis, Vibrational Assignment, and Ab Initio Calculations of 3,3-Difluorobutene. J. Phys. Chem. A 2000, 104, 4383–4393. (17) Frisch, M. J.; Yamaguchi, Y.; Gaw, J. F.; Schaefer, H. F., III; Binkley, J. S. Analytic Raman Intensities from Molecular Electronic Wave Functions. J. Chem. Phys. 1986, 84, 531–532. (18) Amos, R. D. Calculation of Polarizability Derivatives Using Analytic Gradient Methods. Chem. Phys. Lett. 1986, 124, 376–381. 2306
dx.doi.org/10.1021/jp108879u |J. Phys. Chem. A 2011, 115, 2297–2307
The Journal of Physical Chemistry A
ARTICLE
(19) Polavarapu, P. L. Ab Initio Vibrational Raman and Raman Optical Activity Spectra. J. Phys. Chem. 1990, 94, 8106–8112. (20) Chantry, G. W. In The Raman Effect; Anderson, A., Ed.; Marcel Dekker, Inc.: New York, NY, 1971; Vol. 1, Chap. 2. (21) Van der Veken, B. J.; Herrebout, W. A.; Durig, D. T.; Zhao, W.; Durig, J. R. Conformational Stability of 3-Fluoropropene in Rare Gas Solutions from Temperature-Dependent FT-IR Spectra and Ab Initio Calculations. J. Phys. Chem. A 1999, 103, 1976–1985. (22) Durig, J. R.; Ng, K. W.; Zheng, C.; Shen, S. Comparison of Ab Initio MP2/6-311þG(d,p) Predicted Carbon-Hydrogen Bond Distances with Experimentally Determined r0 (C-H) Distances. Struct. Chem. 2004, 15, 149–157. (23) McKean, D. C. New Light on the Stretching Vibrations, Lengths and Strengths of CH, SiH and GeH Bonds. J. Mol. Struct. 1984, 113, 251–266. (24) Bulanin, M. O. Infrared Spectroscopy in Liquefied Gases. J. Mol. Struct. 1973, 19, 59–79. (25) Van der Veken, B. J.; DeMunck, F. R. An Infrared Study of Monomeric and Oligomeric (n = 2, 3, and 4) Hydrogen Chloride in Liquified Noble Gases. J. Chem. Phys. 1992, 97, 3060–3072. (26) Herrebout, W. A.; van der Veken, B. J.; Wang, A.; Durig, J. R. Enthalpy Difference between Conformers of n-Butane and the Potential Function Governing Conformational Interchange. J. Phys. Chem. 1995, 99, 578–585. (27) Bulanin, M. O. Spectroscopy of Molecules in Liquid Noble Gases. J. Mol. Struct. 1995, 347, 73–82. (28) Galabov, B; Kim, S.; Xie, Y.; Schaefer, H. F., III; Leininger, M. L.; Durig, J. R. Conformations of Allyl Amine: Theory vs Experiment. J. Phys. Chem. A 2008, 112, 2120–2124. (29) Schwendeman, R. H.; Jacobs, G. D.; Krigas., T. M. Molecular Structure of Cyclpropyl Chloride. J. Chem. Phys. 1964, 40, 1022–1028. (30) Kirby, C.; Kroto, H. W. Microwave and Photoelectron Study of cis- and trans-Isocyanatoethene, CH2:CHNCO (Vinyl Isocyanate). J. Mol. Spectrosc. 1978, 70, 216–228. (31) Bouchy, A.; Roussy, G. Geometrical and Electronic Structure of Two Conformers of Vinyl Isocyanate by Microwave Spectroscopy. J. Mol. Spectrosc. 1977, 68, 156–65. (32) Yamada, K. Molecular Structure and Centrifugal Distortion Constants of Isocyanic Acid from the Microwave, Millimeterwave, and Far Infrared Spectra. J. Mol. Spectrosc. 1980, 79, 323–346. (33) Penn, R. E.; Bogg, J. E. Substituent-Induced Asymmetry of Cyclopropane Ring. J. Chem. Soc., Chem. Commun. 1972, 666–667. (34) Durig, J. R.; Sullivan, J. F.; Berry, R. J.; Cradock, S. An Electron Diffraction and Ab Initio Investigation of Cyclopropylisothiocyanate. J. Chem. Phys. 1987, 86, 4313–4318. (35) Groner, P.; Church, J. S.; Li, Y. S.; Durig, J. R. Spectra and Structure of Organophosphorus Compounds. XXIII. Microwave Spectra, Electric Dipole Moment and Molecular Structure of Two Conformers of Ethyldifluorophosphine. J. Chem. Phys. 1985, 82, 3894–3902. (36) Winnewisser, M.; Winnewisser, B. P.; Medvedev, I. R.; de Lucia, F. C.; Ross, S. C.; Bates, L. M. The Hidden Kernel of Molecular QuasiLinearity: Quantum Monodromy. J. Mol. Struct. 2006, 798, 1–26.
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dx.doi.org/10.1021/jp108879u |J. Phys. Chem. A 2011, 115, 2297–2307
