J . Phys. Chem. 1984, 88, 374-380
374
not all, of the trend is attributable to the heavier mass of Se and the mixing of S and Se motions into the various porphyrin modes. This can be seen from the product which can be written in the following form: nFi,Se2TPP/nFi,S,TPP
= mSenV,,Se2TPp2/(mSllu,,S2TPP2)
where ms and mse are the S and Se masses, u, and F, are the frequency and force constant for the ith normal mode of the indicated molecule, and the products are taken over all the modes in a given symmetry block. When evaluated for the 15 assigned A, modes of S2TPP and Se2TPP, the right-hand side of the equation has a value of 0.93, implying a 7% reduction in the product of the A, force constants between S2TPP and Se2TPP. The calculation leaves out of account the unobserved A, mode, vl, which is CbH stretching. Since vI does not involve appreciable S(Se) motion, its omission does not affect the conclusion that the porphyrin restoring forces are significantly weakened in Se2TPP, relative to S2TPP. This conclusion directly supports the results of M O calculation^,^ indicating that net a bonding is lower in Se2TPP than S2TPP because Se is more effective than S in draining a electrons from the ring. The modes most directly involving the X atoms are those as) to the symmetric deformations signed to C,X stretching ( q 3 and of the X-rings (u16). As noted above, these are found at frequencies close to the analogous modes of thiophene and selenophene themselves. SSeTPP shows both sets of bands, implying essentially localized motions of the X-rings. Some interaction between the rings is indicated however by the lack of coincidence between the SSeTPP frequencies and the corresponding frequencies for S2TPP and Se2TPP. In particular the C,S and Case frequencies ( q 3 ) are both lower, by 8 and 10 cm-’, than those of S2TPP and Se2TPP. It is possible to explain these lowerings by taking into account the cross-porphyrin X. -X bonding interactions which the M O calculations3 suggest to be significant, particularly for
-
(26) Wilson, E. B.; Decius, J. C.;Cross, P. C. “Molecular Vibrations, The Theory of Infrared and Raman Vibrational Spectra”; McGraw-Hill: New York, 1955.
Se2TPP, which has a short Se.. .Se separation, 2.80 A, compared to 3.04 A for the .S separation in S2TPP.5 An attractive force is implied by the direction of the shifts. However, the required perturbation could arise via the porphyrin conjugation as well as a direct X. -X interaction. Stronger evidence for X. * .X bonding might be the identification of a mode attributable to XX stretching. Disulfides and diselenides show XX stretching bands at -520 and ~ 2 9 cm-’, 0 the frequency lowering being due to the greater Se mass and lower Sese force constant. In porphyrins, however, the number of vibrational modes is completely determined by the structure of the ring and is independent of any cross-ring interactions. Stretching of the XX bond, if present, is redundant with the deformation coordinates of the ring. Nevertheless, the XX interaction, if present, should contribute significantly to the low-frequency deformation modes. In particular, it is reasonable to expect that one of these modes involves a radial motion of the X-rings themselves and would be principally affected by the X- .X interaction. We tentatively identify Y8 (Table I) with this mode, noting the strong decrease from S2TPP(330 cm-I) to SSeTPP (295 cm-I) to Se2TPP (280 cm-’). The decrease, however, is less than might be expected from mass effects alone. If this mode is regarded as being due to a hypothetical diatomic oscillator with dynamical masses equal to those of thiophene (94) and selenophene (141), then the required force constants would be 3.02, 2.90, and 3.25 mdyn/A. If the dynamical masses are just those of S and Se, the force constants would be 1.03, 1.17, and 1.82 mdyn/A. In either case the trend is consistent with stronger Se...Se than S...S bonding. In the absence of a reliable normal-mode calculation, however, one cannot rule out that the trend reflects differential mixing of the ring displacements with other deformation coordinates. S a
-
-
Acknowledgment. This work was supported by NSF Grant CHE-8106084 and N I H Grant H L 12526 (to T.G.S) and by funds provided to P.S. through a N I H Biomedical Research Support Grant, 11H-2940-8751, to Washington State University. Registry No. Se2TPP, 66951-06-2; SSETPP, 66951-07-3; S2TPP, 5751 1-57-6; H2TPP,917-23-7; (FeTPP),O, 12582-61-5.
Conformational Study and Vibrational Analysis of Ethyl Thiocyanate J. R. Durig,* J. F. Sullivan, and H. L. Heuself Department of Chemistry, University of South Carolina, Columbia, South Carolina 29208 (Received: May 25, 1983)
The infrared spectra (3500-40 cm-I) of gaseous and solid CH3CH,SCN and the Raman spectra (3200-10 cm-’) of the liquid and solid have been recorded. The vibrational spectrum has been assigned on the basis of a gauche form of C, symmetry, but several lines observed in the Raman spectrum of the liquid have been attributed to the less stable trans conformer. From a variable-temperature Raman study of the liquid, an enthalpy difference of 586 f 23 cm-I (1.68 f 0.07 kcal/mol) between the gauche and trans conformers was determined by measuring, as a function of temperature, the intensities of the lines attributed to the S-CN stretches of the two conformers. The methyl torsion was observed as a broad, nondescript band at approximately 230 cm-I in the spectrum of the gas which gives a periodic barrier of 3.5 kcal/mol(l220 cm-I). A reassignment of several of the low-frequencymodes has been presented and normal-coordinatecalculations have been carried out for both conformers. The results of this study are discussed in relation to the previous studies on ethyl thiocyanate and compared to corresponding studies of similar molecules.
Introduction Many of the early studies on rotational isomerism have centered on the rotation about carbon-carbon single bonds; however, more attention is now being directed to rotational isomerism about carbon-heteroatom single bonds. We have recently extended our ‘Taken in part from the thesis of H. L. Heusel which will be submitted to the Department of Chemistry in partial fulfillment of the Ph.D. degree.
0022-3654/84/2088-0374$01.50/0
structural and vibrational studies of group 4A compounds with pseudohalogen linkages’ to include those alkyl isocyanates2 and i s o t h i ~ c y a n a t ewhich s ~ ~ ~ can exhibit rotational isomerism about (1) J. R. Durig, M. R. Jalilian, J. F. Sullivan, and J. B. Turner, J. Ramon Spectrosc., 11, 459 (1981), and references therein. (2) J. R. Durig, K. J. Kanes, and J. F. Sullivan, J . Mol. Struct., 99, 61 (1983). (3) J. R. Durig, J. F. Sullivan, and T. S. Little, J . Mol. Struct., in press.
0 1984 American Chemical Society
Study of Ethyl Thiocyanate the C,-N bond. In general, it has been found that the vibrational data on these types of molecules do not yield conclusive evidence for the existence of more than one conformer, presumably because of the rather large angle (- 140-1 50') and amplitudes of vibration moiety. However, alkyl thiocyanates, whose of the C-N=X C-S-C angles are on the order of loo', should exhibit significantly different vibrational spectra for different conformers when they are present in the fluid phases. Several spectroscopic studies have been carried out on CH3CH2SCN and its rotational isomerism. From an early infrared study of the liquid and solid states of CH3CH2SCN,Hirschmann et aLS concluded that the molecule exists in two conformations, the gauche and trans (SCN group trans to the methyl group) forms. From the temperature dependence of the absorbance ratio of two bands observed in the liquid at 686 and 654 cm-', and assigned to the S-C stretch of the trans and gauche conformers, respectively, theyS calculated an enthalpy difference of 0.49 f 0.03 kcal/mol between the more stable trans and high-energy gauche forms. In a subsequent study, Crowder6 carried out a force constant calculation on trans-CH3CH,SCN by combining the infrared data presented by Hirschmann et al.5 with the Raman data obtained by Vogel-Hogler,' and his calculations for the most part support the assignment proposed by Hirschmann et ale5 However, in a more recent microwave investigation,x only the gauche rotational isomer was positively identified from the Stark-effect measurements and, on the basis of the calculated relative intensity of lines of an expected trans conformer, the authors concluded that the gauche form is at least 0.8 kcal/mol more stable than the trans conformer. These authors* could not identify any microwave lines for a second conformer. The discrepancy between the results of the earlier vibrational studySand the microwave investigationx prompted Ellestad and Torgrimseng to reinvestigate the vibrational spectrum of CH3CH2SCN. They concluded that ethyl thiocyanate exists only in the gauche conformation and they attributed the disappearance of the band at 654 cm-' in the spectrum of the solid to a "temperature effect". After a careful analysis of this latest study, we felt that the vibrational spectrum of ethyl thiocyanate should be reanalyzed, particularly in the region below 700 cm-' where the normal modes are most sensitive to conformational changes.
Experimental Section Ethyl thiocyanate was obtained commercially and purified by low-temperature fractionation on a vacuum-sublimation column. The compound was stored over activated 3-A molecular sieves at room temperature, and all subsequent sample transfers were carried out under high-vacuum conditions. Raman spectra were recorded from 3200 to 50 cm-I (liquid phase) and from 3200 to 5 cm-I (solid phase) by using a Cary Model 82 Raman spectrophotometer equipped with a SpectraPhysics Model 171 argon ion laser tuned to the 514.5-nm emission line. The spectrum of the liquid was obtained at room temperature from the sample sealed under vacuum in a glass capillary. Depolarization measurements were made by using the standard Cary accessories. A temperature study of the liquid phase of CH,CH,SCN was also conducted by utilizing the method of Miller and Harney.lo The Raman spectrum of the solid phase was recorded by condensing the sample onto a blackened brass block positioiied at an angle 15' from the normal and maintained at -77 K with boiling nitrogen, and annealed until no further changes were noted in the spectrum. Typical spectra are shown in Figure 1, and the frequencies for sharp resolvable lines should be accurate to f 2 cm-'. (4) J. R. Durig, A. B. Nease, J. F. Sullivan, Y. S. Li, and C. J. Wurrey, J. Chem. Phys., in press. ( 5 ) R. P. Hirschmann, R. N. Kniseley, and V . A. Fassel, Spectrochim. Acta, 20, 809 (1964). (6) G.A. Crowder, J . Mol. Struct., 7 , 147 (1971). (7) R. Vogel-Hogler, Acta Phys. Austriaca, 1, 31 1 (1948). (8) A. B j ~ r s e t hand K. M. Marstokk, J . Mol. Struct., 11, 15 (1972). (9) 0. H. Ellestad and T. Torgrimsen, J. Mol. Struct., 12, 79 (1972). (10) F. A. Miller and B. M. Harney, Appl. Spectrosc., 24, 291 (1970).
The Journal of Physical Chemistry, Vol. 88, No. 3, 1984 375
I
1
II
I
3000
A
lobo
20b0
0
WAVE N U MB E R ( c d ) Figure 1. Raman spectra of CH3CH2SCNin the (A) liquid and (B) solid phases.
I
1
1
1
1
I
I
I
1
1
1
1
I
2500
1
1
,
I
1
I
I 3000
1
1
1
I
2000
I
I
I
I
1500
,
lob0
, I
500
WAVENUMBER (CM-')
Figure 2. Mid-infrared spectra of CH3CH2SCNin the vapor (upper trace) and solid (lower trace) phases.
Mid-infrared spectra of the gaseous and solid phases were recorded between 3500 and 400 cm-I by using a Digilab Model FTS- 14C Fourier transform interferometer equipped with a high-intensity Globar source, Ge/KBr beam splitter and a TGS detector. Atmospheric water vapor was removed from the spectrometer housing by purging with dry nitrogen. The midinfrared spectra of CH3CH2SCNare shown in Figure 2. The spectrum of the gas was recorded with a theoretical resolution of 1.0 cm-' by using a 10-cm cell equipped with CsI windows. A low-temperature cell equipped with CsI windows was used to record the spectrum of the solid phase; conventional vacuumsublimation techniques were employed in which a solid film of sample was deposited onto a CsI substrate held at -77 K by boiling nitrogen. The sample was annealed until no further changes were noted in the spectrum. The spectrum of the solid was recorded at an effective resolution of 2 cm-'. The far-infrared spectra of the gaseous and solid phases were recorded on a Digilab FTS-15B Fourier transform interferometer
316
The Journal of Physical Chemistry, Vol. 88, No. 3, 1984
500
300
Durig et al.
100
WAVE NUMB ER (cm'l)
Figure 3. Far-infrared spectra of CH,CH2SCN in the vapor (upper trace), amorphous solid (middle trace), and annealed solid (lower trace).
equipped with a high-pressure Hg arc lamp source and a TGS detector. The spectrum of the solid was obtained by using a cold cell equipped with polyethylene windows, in which the sample was deposited onto a silicon plate cooled by boiling nitrogen. The spectrum was recorded after 500 scans at an effective resolution of 2 cm-' with 6.25- and 12.5-fim Mylar beam splitters. The sample was annealed until no further changes were observed in the spectrum. The far-infrared spectra of gaseous CH3CH2SCN was recorded from 500 to 70 cm-' with a 6.25-pm Mylar beam splitter. Atmospheric water vapor was removed from the spectrometer housing by purging with dry nitrogen. Typical spectra are shown in Figure 3.
Results From an inspection of the Raman data for the liquid phase of ethyl thiocyanate (Table I), it can be seen that nearly all of the observed lines are polarized and, therefore, the data have been assigned on the basis of a more stable gauche conformer with C1 symmetry. Our assignments are in agreement with the previous assignment9except for the carbon-hydrogen stretching region and the region below 700 cm-l where the existence of a second conformer, if present, should be apparent. We believe that the 2875-cm-' line, which has previously been a s ~ i g n e dto~ ~the ~ ,CH3 ~ symmetric stretch, is a combination or overtone of the CH3 antisymmetric deformations in Fermi resonance with the CH3 symmetric stretch at 2947 cm-'. Additionally, we have assigned the CH, antisymmetric stretch to the highest frequency line, based on its weaker intensity. The two CH3 antisymmetric stretches appear to be at the same frequency in the gas and liquid phases; the degeneracy may be lifted in the solid but splitting due to crystal effects cannot be ruled out entirely. In the frequency region below 700 cm-' one expects two stretches, four bends and two torsions. The C-S-C bend and the asymmetric torsion are expected to be the lowest frequency modes and fall below 200 cm-I. It should be noted that, in the previous s t u d i e ~ ,the ~ ~authors ~ , ~ described the normal modes in such a way that the misassignment of the asymmetric torsion is not apparent. In other words, we have assigned only one C-C-S bend; its counterpart under C, symmetry may be called an "out-of-plane" C-C-S bend but, in essence, either the CCS out-of-plane bend or the CSC out-of-plane bend is better described as the asymmetric torsion, the frequency of which is expected to be below 100 cm-l. Our assignment of the low-frequency region is more consistent
500
250
WAVE Nl J MB E R ( c m-') Figure 4. Raman spectra of CH3CH,SCN at (A) 27 and (B) -54 OC.
with the assignment reported by Kniseley et al.,5 even though it was for the trans conformer, rather than the one presented by Ellestad and Torgrimsen.' In the latter study, the authors assigned bends to frequencies lower than the C-C-S bend. the S-C%N However, in spectra which we have recorded" for CH3SCN, the S-CEN bends are unquestionably at frequencies of 462 (inplane) and 408 (out-of-plane) cm-', and it is unlikely that these frequencies would differ significantly for CH3CH2SCN. The presence of conformers is apparent in the frequency region below 700 cm-I where three lines at 652, 457, and 316 cm-I are present in the spectrum of the liquid but absent in the spectrum of the solid. These bands were also observed in the previous ~ t u d i e s ,but ~ . ~they were either misassigned or not assigned at all. Our normal-coordinate calculations, which will be discussed in more detail in another section, were carried out for the gauche form of ethyl thiocyanate and the resulting force constants were then used to predict the frequencies for the trans conformer (Table 11). For most of the vibrations, there is little difference between the frequencies of the gauche and trans normal modes. However, the heavy-atom vibrations do show significant frequency differences, particularly the S-CN stretch (v18)at 626 cm-'. Kniseley et al.,5in their vibrational study, assigned the 652-cm-I line as the S-CN stretch of the less stable conformer, and the corresponding vibration for the more stable conformer at a higher frequency of 686 cm-I. However, our normal-coordinate calculations indicate that the corresponding vibration for the more stable conformer is, in fact, at a lower frequency of 626 cm-I. Additionally, in the 400-cm-' region of the Raman spectrum of the liquid, we observed three lines even though only two are expected. Initially, we attributed the 468-cm-' line to the trans conformer based upon its intensity as compared to the 457-cm-' line. (1 1) H. L. Heusel, Ph.D. Thesis, University of South Carolina, Columbia, SC, 1983.
The Journal of Physical Chemistry, Vol. 88, No. 3, 1984 377
S t u d y of Ethyl Thiocyanate
TABLE I: Observeda Infrared and Raman Frequencies (cm-' ) and Vibrational Assignment for Ethyl Thiocyanate Raman infrared gas
re1 int and Liquid depol
re1 int
calcd
2998 2994
s sh
3003
ii,
vs
s, sh, p
2977 2971 2949
vs vs
2976 2975 2943
CH, antisymmetric stretch (99%) v2 J ' ~ CH, antisymmetric stretch (98%) u4 CH, symmetric stretch (96%)
vs, p
2934
vs
2934
J ) ~
solid
re1 int
vs
2992
s
2992 R 2984Q 2978 P
vs
2975 2968 2946
vs vs ni
2973 2948
2932
vs
2934
2916 2893 2873 2847 2837 2739 2153
m m s s s m vs
2198 2125 2105
m w
s
1457
s
s
1450 1446 1434 1430
vs
1450
m,dp?
s vs
1430
m,p
1380
vs
1382
1274
vs
1247 1242
2891 2857 2176Q 2171 Q 2165 Q 2081 1471 R 1466Q 1457Q 1451 P 1435Q 1402 R 1395Q 1389Q 1383 P 1289 R 1283Q 1273Q 1264 P 1071 R 1065 Q 1050 P 979 R 971 Q 965 P 773 713 -641 -630
vs
m w s s
s m
m
J J ~
CH, antisymmetric stretch (99%)
CH, symmetric stretch (97%) 2Ji7 2J),
2875
s,p
2872
m
'8
+
v9
2v9
2846 2740 2155
w w vs
vw,p
2125 2106
vw w
1460
m
1467
v7
1454 1447 1438 1433
m m,sh m m
1464
tig
1432
vQ
CH, deformation (64%); CH, wag (19%);CH, antisymmetric deformation (1 1%)
w,p
1386
w
1424
vIu
CH, symmetric deformation (74%): CH, deformation (12%);CH, wa8 (12%)
1273
m,p
1276
m
1280
vll
CH, wag (57%); CH, symmetric deformation (25%); CH, deformation (14%)
s, sh vs
1245
w,p
1246
m
1243
iil,
CH, twist (97%)
1067 1062 1056
vs
1062
m,p
CH, rock (77%); CH, rock (14%)
w, sh, p
m m m
J ) ~ ,
1050
1068 1063 1057
1056
s m
1048
v14
CH, rock (69%); C-C stretch (7%);CH, wag (7%)
968 962 837 775 726 685
vs
968
w,p
C-C stretch (77%)
s, sh w s m
818 777
vw,p w,p
631 628 475
s
686 652 626
w,p m,p s,p
m
2840 2740 2154
'6
2171
vo
"17
2VlO C-N stretch (93%) residual CH,CH,NCS
2095
s
"13
+
'14
2'1, CH, antisymmetric deformation (8 1%);CH, rock (9%); CH, deformation (8%) CH, antisymmetric deformation (92%)
m m
s
s
m
m
m m
s
w, sh vw
vw, sh, p w,p vw,dp m,p
vw',sh vw w
316 255
165
w
157
120 90
w w
-302 -230
vvw ww
(327) 272 182
vw
ww
s, sh
468 457 410 331
vu' vu! w
71
s,p
approx descripn
___-
m,p m,p vs, p
-480 446 395 Q -315
154 R 148Q 142 P 138Q
assignment and PED
solid
re1 int
2999Q
2955 R 2947 Q 2940 P
-
417 334
s m vw
971 963 837 778
m w,sh vw w
971
1iI5
774
v,6
686
m
687
vl7,
633 628 479
s
625
iilH
468
'19,
'I8
s, sh
w
'19
416 337
w
m
410 331
w,sh,p w,bd,?
274
w
255
v,,
s,p
168
m
158
u2,
'ZII
u,, ' 2 I
vh)
vvw
'23
64
vw
108 68 40
2v,,, CH, rock (73%);CH, rock (10%) HCN impurity C,-S stretch (49%); S-CN stretch (26%) trans conformer S-CN stretch (61%); C,-S stretch (22%)
s
s vs
75
'24
SCN bend (65%); CSC bend (21%) trans conformer SCN bend (95%) CCS bend (54%); C-C stretch (14%); C,-S stretch (13%); SCN bend (9%) trans conformer CH, torsion (90%) two-phonon band
CSC bend (69%); SCN bend (16%); CH, torsion (8%) trans conformer lattice mode asymmetric torsion (9 1%) lattice modes
a Abbreviations used: sh, shoulder; dp, depolarized; p, polarized; v, very; s, strong; m, medium; w, weak. Parentheses indicate bands observed in the unannealed solid but absent in the spectrum of the annealed solid.
378 The Journal of Physical Chemistry, Vol. 88, No. 3, 1984
Durig et al.
TABLE 11: Observed and Calculateda Frequencies (cm-I) for Gauche and Trans CH,CH, SCN from the Normal-Coordinate
Calculations gauche l'i Ul
ti 2 1) 3 114
v5
I)(> 7' "8
"9 '11)
Ul.
"12 '13 "I4 '15 "16 '17
"1 R "1 9 '120
2' I lizz "23 '24
obsd
calcd
2999 2975 2968 2946 2932 2176 1466 1457 1435 1392 1278 1242 1065 1050 971 773 685 631 468 395 331 255 148 71
3004 2976 2975 2943 2934 2171 1467 1464 1432 1424 1280 1243 105s 1048 972 714 687 625 46 8 410 33 1 255 158 75
__ obsdb
652 457 316 138
trans
calcd 3003 2976 2975 2943 2934 2171 1470 1464 1444 141 1 1207 1257 1056 1040 97 1 765 685 650 45 3 408 318 25 1 146 79
T(%) 25' .16' .32' -64' .a4* Figure 5. Variable-temperaturestudy of the Raman spectra of liquid CH,CH2SCN from 700 to 550 cm-' (at -84 "C,the sample is in the solid
phase).
I
1.5
1.3+
Calculated by using the force constants listed in Table IV. Observed frequencies not listed for the trans conformer are assumed to be the same as the corresponding frequencies for the gauche. a
I'
However, our normal-coordinate calculations indicate that the trans SCN bend is at a lower frequency than the gauche SCN bend (vI9). We, therefore, examined this region at various temperatures and in Figure 4 one can see that, as the temperature is decreased, the 457-cm-' line decreases in intensity whereas the very weak shoulder at 468 cm-I increases. Similarly, the trans CCS bend is predicted at 318 cm-' and in the Raman spectrum of the liquid there is a weak shoulder at 316 cm-l which is absent i n the spectra of the annealed solid, as shown in Figure 3. The CSC bend appears at 148 cm-' in the infrared spectrum of the gas and as a strong, polarized line at 157 cm-I in the Raman spectrum of the liquid. It is surprising that we did not observe the corresponding vibration for the trans conformer in the Raman spectrum, which is predicted to be about 10 cm-' lower in frequency, but it may be that the frequency differential is smaller and the two modes are essentially overlapped. In the infrared spectrum of the gas there are several Q branches between 148 and 144 cm-' which are probably due to "hot bands" associated with the asymmetric torsion but an additional weak Q branch was observed at 138 cm-l which may arise from the CSC bend of the trans conformer. The assignment of the 182-cm-I band, observed in the infrared spectrum of the solid, to the trans CSC bend was considered; however, because its frequency is predicted to be lower than the gauche CSC bend and because this band exists in the spectrum of the annealed solid, we feel that it may be a two-phonon band (120 64 = 184 cm-'). The asymmetric torsion gives rise to a very weak, nondescript band at approximately 7 1 cm-' and it may correspond to the band at 90 cm-' in the spectrum of the solid.
+
A H in the Liquid As discussed in the previous section, a comparison of the Raman spectra of the liquid and solid phases of CH,CH,SCN reveals the presence of several trans and gauche conformer bands. To determine the enthalpy difference between the conformers, a variable-temperature Raman study of the liquid was performed. These measurements were achieved by assuming that the intensity of the lines due to the two different conformers is temperature dependent and will change as the temperature varies.
3.4
3.8
4.2 1/Tx103
4.6
Figure 6. Temperature dependence of the ratio of trans to gauche conformers for ethyl thiocyanate. TABLE 111: Temperature and Intensity Ratios (652/626-cm-I Lines) for the Conformational Study of Ethyl Thiocyanate T, "C
lOOO(l/T), K - '
K=I,/Ig
- II1K
2s 3 -6 -28 -44 -61
3.35 3.62 3.74 4.08 4.36 4.71
0.68 0.53 0.4 1 0.34 0.28 0.21
0.39 0.63 0.90 1.09 1.28 1.57
As can be seen from the spectra (Figures 4 and 5), two sets of lines which show significant changes upon solidification are at 626 and 652 cm-' and at 468 and 457 cm-'. The first set of lines has been assigned to the S-CN stretch of the gauche (626 cm-') and trans (652 cm-I) conformers, based on our normalcoordinate calculations. Because these bands are better resolved and more intense than those of the second set, they were chosen for the temperature study. By applying the equation -In K = (AH/RT)- ( S I R ) ,where K is the ratio of trans to gauche states and AS is the change in entropy, one can determine AH by making a plot of - In K vs. 1 / T where A H / R is the slope of the curve. In the case of the pure liquid, K is the ratio of Zt/Zg where Z is the intensity of the Raman line. Six sets of low-temperature spectral data were obtained at temperatures ranging from +25 to -86 "C (Table 111). The ratios of the intensities of the two lines were plotted as a function of the reciprocal of the absolute temperature (Figure 6). A value of 5 8 6 f 23 cm-l (1.68 0.07 kcal/mol) was obtained for AH based on the slope of the curve. Normal-Coordinate Analysis As stated previously, we carried out a normal-coordinate calculation based on the more stable gauche conformer of ethyl thiocyanate. The resulting force field was then used to predict
The Journal of Physical Chemistry, Vol. 88, No. 3, 1984 379
Study of Ethyl Thiocyanate TABLE IV: Valence Force Constants for Ethyl Thiocyanate force constant
KX KQ KR KS K, Kd Hg
HQ' HE Hz' H, HD
Hg H, He H, HT8 F,, Fxr Fge
Fxp
Fxa a
description
value,a mdyn/A
C-C stretch C-S stretch S-C stretch e N stretch C-H stretch (-CH,) C-H stretch (-CH,) S-C%N bend S--C%N bend C-C-Sbend C-S-C bend H-C-H bend (-CH,) H-C-C bend (-CH,) H-C-H bend (-CH,) C-C-H bend (-CH,) H-C-S bend CH, torsion asymmetric torsion C-H/C-H stretch (-CH,) C-C stretch/C-C-H bend H-C-H/H-C-S bend C-C stretch/H-C-C bend C-C stretch/H-C-H bend
4.03 i 0.02 2.56 t 0.02 3.75 i 0.03 16.75 i 0.03 4.79 iO.O1 4.82 I0.01 0.43 = 0.01 0.40 5 0.01 1.25 i 0.03 0.75 f 0.02 0.54 t 0.01 0.68 i 0.01 0.55 i 0.01 0.53 f 0.01 0.83 t 0.01 0.01 i 0.01 0.008 +- 0.001 0.09 i 0.01 0.55 t 0.02 0.05 i 0.01 0.10 t 0.01 -0.30 0.01
*
The bending coordinates are weighted by 1 A .
the frequencies for the trans conformer. The following structural parameters for gauche-CH3CH2SCNwere taken from those determined in the microwave study: r(C-H) = 1.091 A; r(CB-C,) = 1.540 A; r(C,-S) = 1.820 A; r(S-C) = 1.690 A; r(C%N) = 1.160A;LHCBH= 109.S0;LHC,H = 1l1.3';LCBC$ = 1 12.0°; LC,SC = 101.0'; LSCN = 180.0'; and the dihedral angle = 122.0' (from the trans structure). The 26 internal coordinates used to construct the symmetry coordinates for CH3CHzSCN are similar to those previously reported.12 In the above case, the perturbation program developed by Schacht~chneider'~ was used to adjust the force constants, thereby obtaining the best fit for the frequencies and the potential energy distribution. The calculated vibrational frequencies of gaucheCH3CH2SCN are listed in Table I along with the associated potential energy distribution. The force field, consisting of 17 diagonal force constants and 5 interaction terms, is given in Table IV. The observed frequencies were reproduced by these force constants with an average error of 0.3%. In our initial calculations, the force constants were transferred from CH3CH2NCSI2and CH3SCN.I1 In the final force field, the force constants for the ethyl moiety are in good agreement with those found in CH3CH2NCS.12 The potential energy distribution indicates that a small amount of vibrational coupling exists among the CH2 deformation, CH3 symmetric deformation, and the CH2 wagging vibrations and among the C-C stretching and CH3 rocking vibrations. To improve the frequency fit of the vibrations of the ethyl moiety, an interaction was added between the HCH(CH2) and HC,S bends along with interactions between the C-C stretch and CCH(CH,), HCC(CH3), and HCH(CH3) bends. The only extensive mixing occurs between the C-S and S-C stretching frequencies. Since no interaction satisfactorily separated the two stretches, the two bands are more accurately described as the antisymmetric and symmetric C-S-C stretches. In order to obtain the best fit for the C-H stretching vibrations, a small interaction was required between the two C-H(CH3) stretching force constants. It is difficult to compare the results of our normal-coordinate calculations with the previous results6 because different assignments were used for the low-frequency modes as well as frequencies from the condensed states. Additionally, the earlier force field was based on a more stable trans form. (12) J. R. Durig, J. F. Sullivan, H. L. Heusel, and S. Cradock, J . Mol. Struct., in press. (1 3) J. H. Schachtschneider, "Vibrational Analysis of Polyatomic Molecules, Parts V and VI", Technical Report Nos. 231 and 57, Shell Development Co., Emerwille, CA, 1964 and 1965.
Discussion The vibrational spectra of ethyl thiocyanate have been recorded and changes in the spectral data have been observed among the three phases. It has been concluded that CH3CH2SCNexists in two conformations in the liquid phase, the more stable gauche form and the high-energy trans form. In the solid phase, the spectral data indicate that only the gauche conformer exists. It is difficult to ascertain whether more than one conformer is present in the gas phase because of the breadth of the bands. However, there is a strong indication that the infrared band associated with the S-CN stretch has two maxima at 630 and 647 cm-'. Also there is some evidence of a second maximum for the infrared band assigned as the CCS bend along with a second Q branch at 138 cm-' for the CSC bend for the trans conformer. The enthalpy difference between the more stable gauche and high-energy trans conformers in the liquid phase has been determined to be 1.68 f 0.07 kcal/mol (586 f 23 cm-'). This value is much larger than the value of 0.49 f 0.03 kcal/mol reported by Kniseley et al.,5 whose choice of conformers bands was incorrect. Their choice of the 686-cm-' band would lead to a smaller value of the AH since this band is due to the C,-S stretch of both conformers. From our large AH value, it is estimated that only about 10% of the molecules exist in the trans conformation in the liquid phase. It is possible that the AH for the gas phase may even be higher because we have determined the AH values in both the gas and liquid phases for a number of molecules and, in most of the cases, the gas-phase values are considerably larger. We believe that the differences between the gas- and liquid-phase AH values are due to dipole-dipole interactions in the liquid phase along with a preference for the more symmetric form in the liquid state. Thus, these results are consistent with the microwave resultss in that only a very small amount of the trans conformer is present in the gas phase. In contrast to the present study, only one conformer has been found in the vibrational spectra of the ethyl isocyanate,14 ethyl isothiocyanate,12 isopropyl isocyanate,2 and isopropyl isothiocyanate3 molecules in the fluid phases. However, all of these molecules have very large C-N=C angles (>138') and it may well be that there are not many "bound" states below the linear configuration. From the microwave spectra of vinyl i~ocyanate'~J~ and cyclopropyl is~thiocyanate~ it has been shown that both of these molecules have two conformers present in the gas phase (Le., cis and trans) but their energy differences are very small. Certainly the lone pair on nitrogen and the formal double bond between the nitrogen and carbon atoms make it difficult to compare the conformational preference for these molecules with the thiocyanate molecules. Therefore, we believe the thiocyanate molecules with their much smaller CSC angles, as opposed to the large CNC angles in alkyl isocyanates and isothiocyanates, are unique and it is thus possible to detect the second conformers in the fluid phases from their vibrational spectra. Even though Kniseley et aL5 utilized the incorrect bands for their AH determination, it is clear that the relative changes of the low-frequency bands are due to conformational populations. Thus, it is very probable that their conclusion that isopropyl thiocyanate also exists in the fluid phases as a conformer pair is correct. However, it is probable that the AH value that they obtained for this molecule is also in error since they have compared the relative intensities of two bands which represent different normal vibrations for the two conformers. Therefore, the temperature dependence of the vibrational spectrum of this molecule should be reinvestigated. Several of the normal modes in the solid show factor group splitting. In most of the cases they are doublets, and the frequencies observed in the Raman and infrared spectra are the same within experimental error, which indicates that there are two molecules per primitive cell. There are no X-ray crystal structural determinations with which to compare these results. However, ~
~
~
~
~~
~-
~
~
(14) D. T. Durig, private communication. (15) A. Bouchy and G. Roussy, J . Mol. Spectrosc., 68, 156 (1977). (16) C. Kirby and H. W. Kroto, J. Mol. Spectrosc., 70, 216 (1978).
380
J . Phys. Chem. 1984, 88, 380-381
it should be noted that the number of observed lattice modes is consistent with this conclusion. The methyl torsion was observed as a broad, nondescript band at -230 cm-I in the infrared spectrum of the gas. By utilization of this frequency along with an F-number ( F = h2/8r2Z,where I, is the reduced moment of inertia for the internal rotation) value of 5.37 cm-I, a threefold periodic barrier of 3.5 kcal/mol (1220 cm-I) was calculated. This value agrees very well with the corresponding barriers in ethyl chloride, bromide, and iodide.17 The
barrier in the liquid was calculated to have a value of 4.2 kcal/mol which is raised to 4.9 kcal/mol in the solid state. This change in barrier with physical state is not unusual for molecules which have significant dipoledipole interactions in the condensed states.18
Acknowledgment. We gratefully acknowledge the financial support for this study by the National Science Foundation, Grant CHE-82-15492. Registry No. Ethyl thiocyanate, 542-90-5.
(17) J. R. Durig, W. E. Bucy, L. A. Carreira, and C. J. Wurrey, J . Chem. Phys., 60, 1754 (1974).
(18) J. R. Durig, S. M. Craven, and W. C. Harris in “Vibrational Spectra and Structure”, Vol. 1, J. R. Durig, Ed., Marcel Dekker, 1972, Chapter 4.
Refined Vibrational Data for H20 Isolated in D20 Cubic I c e John E. Bertie and J. Paul Devlin* Department of Chemistry, University of Alberta, Edmonton, Alberta T6G 2G2, Canada (Received: May 25, 1983)
There has recently been a rapid and significant advance in the structural and dynamical modeling of the condensed phases of water (Rice, Whalley, and others). To an appreciable extent this advance has depended on the availability of relatively complete vibrational data for the internal modes of ice (H20 and D20) as well as for the isotopically decoupled frequencies of D20, HOD, and H 2 0 isolated in ice matrices. Of these data the positions of v1 and v2 for H 2 0 isolated in D20 ice have been assigned with the least confidence. In this work the FT IR data required for the assignment of v, and u2 of isolated H 2 0 have been reevaluated at 90 K in a different spectroscopic laboratory and, also, at a lower temperature (15 K). The reduced temperature and the use of slightly higher dilution ratios have permitted a somewhat clearer observation of the isolated molecule spectrum, but basically the tentative values for v1 and v2 have been affirmed. The suggested values for vi, v2, and v g are 3215, 1740, and 3262 cm-’ at 15 K and 3225, 1735, and 3270 cm-’ at 90 K.
Introduction Vibrational data for D 2 0 and H20isolated intact in cubic H 2 0 and D 2 0 ices have become available in recent years’s2 with the results for the internal modes of isolated D 2 0 relatively complete and definitive. Because the v1-u3 region for decoupled H 2 0 is strongly overlapped by the vOH band of contaminant HOD, and the very weak v2 band of H 2 0 underlies the “association” band of P20,some uncertainties have remained in the assignment of the H 2 0 fundamental frequencies. Nevertheless, FT IR subtraction methods have revealed bands assignable to each internal mode: 3200, 3270, and 1732 cm-I for u,, u j , and v2, respectively, at 90 K. The value for u2 is controversial since it is at least 70 cm-’ above more generally accepted values. The correct value for v2 of isolated H 2 0 is of particular concern since Fermi resonance of 2v2 with v 1 is expected to influence the stretching mode spectra for both isolated HzO or D 2 0 and pure H 2 0or D 2 0 ice. The model calculations of the vibrational spectra of ice by Bergren and Rice indicate that, in fact, Fermi resonance is much more significant for D20, than for H,O, ice if the values published for v2, 1230 and 1732 cm-I, respectively, are a ~ c e p t e d . ~ On the other hand, the experimental values for v, (3270 cm-’) and v1 (3200 cm-l) of isolated H 2 0 require a large downshift in the v1 value from resonance with 2v2,since only a 4-cm-’ difference ~ these reasons, there for v3 and v I is expected o t h e r ~ i s e . ’ ,For has been an interest in repeating the infrared measurements of H 2 0 isolated in cubic D 2 0 in an independent laboratory environment. Further, since band narrowing facilitates resolution of the 0-H stretching band system into components, the extension of the infrared measurements to 15 K was also anticipated. This ‘Address correspondence to this author at the Department of Chemistry, Oklahoma State University, Stillwater, OK 74078.
0022-3654/84/2088-0380$01.50/0
paper describes the results of such a repetition of the FT IR spectroscopic study of H20 isolated in D 2 0 ice, with particular attention to results at 15 K. Experimental Section Samples of H20isolated in D 2 0 cubic ice, with the H20content ranging from 5 to lo%, were prepared by codeposition from separate gas lines. An ultrathin film of crystalline D 2 0 ice was deposited at 170 K prior to the codeposition which was made at 125 K. Epitaxial growth at 125 K resulted in H 2 0 isolated in crystalline cubic D 2 0 ice (which also contained a few percent HOD). The deposit substrate, CsI, was supported on the cold tip of an Air Product’s Displex CS-202 refrigerator with temperature control and regulation, to f 2 K, provided by an Artronix controller for the range 15 to 170 K. The infrared spectra were recorded on a Nicolet 7199 FT IR spectrometer at 1.0-cm-I nominal resolution and with HappGenzel apodization. In separate experiments reference spectra for subtraction were obtained of HOD isolated in D 2 0 and of “pure” D20. Prior to any experiments with H 2 0in D 2 0 ice, a series of measurements was made of D 2 0 isolated in HzO to establish that the isolation technique was satisfactory. Results and Discussion In brief, the new data affirm the original published results for H 2 0 isolated in D20.’ The affirmations include the following: (1) G. Ritzhaupt, W. B. Collier, C. Thornton, and J. P. Devlin, Chem. Phys. Let?., 70, 294 (1980). (2) (a) G. Ritzhaupt, C. Thornton, and J. P. Devlin, Chem. Phys. Lett., 59, 420 (1978); (b) G. Ritzhaupt and J. P. Devlin, ibid., 65, 592 (1979). (3) M. S. Bergren and S. A. Rice, J . G e m . Phys., 77, 583 (1982).
0 1984 American Chemical Society
