J. Phys. Chem. 1994, 98, 1416-1420
1416
An MP2/6-31G*//MP2/6-31G* Vibrational Analysis of s-trans- and s-cis-Acryloyl Fluoride, CH*=-CH--CF=O George R. De Mare’ and Yurii N. Panchenko’ Laboratoire de Chimie Physique Molkculaire CP.160/09, Facultk des Sciences, Universitk Libre de Bruxelles, 50 avenue F.-D. Roosevelt, B-1050 Brussels, Belgium
Charles W. Bock Chemistry Department, Philadelphia College of Textiles and Science, Philadelphia, Pennsylvania I9144 Received: October 28, 1993”
The MP2/6-3 lG*//MP2/6-3 l G * harmonic force fields a r e calculated for s-trans- and s-cis-acryloyl fluoride. The scale factors for the quantum mechanical force field of s-trans-acryloyl fluoride are optimized using the experimental vibrational frequencies. The set of scale factors obtained is then transferred to the computed MP2/6-31G*//MP2/6-3 l G * force field of thes-cis rotational isomer. Thevibrationalfrequenciesarecomputed using the corresponding scaled quantum mechanical force fields and compared with experiment. In general, recent reassignments of the vibrational spectra of the acryloyl fluoride rotamers, based on the RHF/6-3 1G/ /RHF/6-31G level calculations, are confirmed (the exceptions being v7 for the s-trans and u7 and vg for the s-cis conformers).
Introduction A recent analysis of the structures and the infrared (IR) and Raman spectra of s-cis- and s-trans-acryloyl fluoride,2 using ab initio molecular orbital techniques, suggested a reassignment of several of the fundamental vibrational f r e q u e n c i e ~ . ~The . ~ study2 was performed at theRHF/6-31G//RHF/6-31Gcomputational level and used scaling technique~s-~ to empirically correct the quantum mechanical force fields. The theoretical justification of this method and theconditionsof its application wereconsidered in ref 8. It was shown, for the ideal molecular model, that the relationship between the exact nonrelativistic and Hartree-Fock (HF) limit values of the quadratic force constants is connected by a single multiplicative (scale) factor. The square root of this factor is the coefficient of the H F determinant in a complete configuration interaction expansion. The approximate fulfillment of the conditions formulated and considered in ref 8 brings about the necessity to use several scale factors for different modes of vibration. Of course, to some scientists, this circumstance results in a sort of “devaluation” of the quantum mechanical calculations. However, these effective harmonic force fields are now widely used to analyze some classes of molecules. Indeed, this approach has been successfully applied to the vibrational analysis of many other conjugated molecules, including buta- 1,3-diene,1°isoprene,’ 2,3-dimethylbuta- 1,3-diene,12 trans,trans,trans- and trans,cis,trans-hexa- 1,3-5-triene,13 all-trans-octa- 1,3,5,7-tetraene,14 gly0xal,15 acrolein,l5 and 2-propen- l-imine.16 These studies have revealed (1) that empirical scale factors can be transferred among related molecules provided that all the calculated force fields are obtained at the same computational leve1,I7J8 and (2) that nonadiabatic effects in such calculations are generally quite small.19 In ref 2, all the required scale factors were transferred directly from related smaller molecules where the vibrational assignments are more reliable. In this sense, the frequencies and the reassignments given in ref 2 can be considered as predicted. However, the scale factors employed in that study, which range from 0.67 to 0.90,2 are somewhat further away from unity than one would prefer if there is any discrepancy between the assignments suggested from computational studies and those from
* Fax: 8
x-32-2-650-4232. Phone: x-32-2-650-4088. Abstract published in Advance ACS Abstracts, December 15, 1993.
0022-3654/94/2098- 1416$04.50/0
,a-
/
H4
c2
\\05
Figure 1. Atom numbering used throughout the text.
experiment. Admittedly, some of the fundamentals reassigned in ref 2 involve overlapping bands in the experimental s p e c t r a ? ~ ~ which complicated the experimental analysis. Consequently, we decided to repeat the vibrational analysis of acryloyl fluoride a t a higher computational level which takes into account the effects of electron correlation. It has been shown that second-order Maller-Plesset perturbation theory (MP2) using a split-valence basis set which includes d-type polarization functions on the heavy atoms can adequately predict thevalues of the experimental band frequencies for vibrations with low anharmonicity (see ref 20, for example). The possibility of obtaining nearly apriori assignments of the vibrational bands in the spectral region which is most difficult to interpret prompted this investigation.
Method and Results Both the planar s-transand s-cis structures of acryloyl fluoride were optimized at the MP2/6-3 1G* level2’ with the GAUSSIAN90 program.22 The correlations were carried out with no frozen core orbitals. The computed geometrical parameters are collected together in Table 1 with the corresponding experimental data and results of previous calculations. The Cartesian force constants were calculated at the MP2/ 6-3 lG*//MP2/6-31G* level using analytical first derivatives and numerical second derivatives. They were then transformed into local (valence) symmetry coordinates25 (Table 2 and Figure 1). Thecomputedvibrational frequencies (seeTable3), although still slightly larger than the experimental frequencies, provide strong a priori evidence for the assignments of the s-trans conformer given in ref 2, as do the computed IR intensities (except 0 1994 American Chemical Society
The Journal of Physical Chemistry, Vol. 98, No. 5, 1994 1417
s-trans- and s-cis-Acryloyl Fluoride
TABLE 1: Structural Parameters and Total Energies of s-trans- and s-cis-Acryloyl Fluoride (angstroms, degrees, and hartree) s-rrans-CHZ=CH-CF=O
s-cis-CH2=CH-CF=O
calcd
calcd exptl
parameter
3-21G ref 23 ref 24 ref4
6-31G ref 2
6-31G* ref 4
MP2/ 6-31G* ref2
MP2/ 6-31G* this work
exptl
- 3-21G ref 23 ref 4
6-31G* ref4
MP2/ 6-31G* ref 2
MP2/ 6-31G* this work
r(C=O)# r(C-C) r(C=C) r(C-F) r(C-H7) r(C-H8) r(C-H)
1.18 1.49 1.35 1.35
1.195 1.186 1.191 1.470 1.463 1.461 1.316 1.325 1.350 1.360 1.376 1.085 1.071 1.072 1.085 1.071 1.071 1.085 1.069 1.071
1.171 1.475 1.320 1.325 1.075 1.074 1.074
1.201 1.470 1.338 1.366 1.084 1.083 1.085
1.201 1.470 1.338 1.367 1.084 1.083 1.085
1.18 1.48 1.35 1.35
1.192 1.468 1.325 1.373 1.072 1.073 1.070
1.171 1.477 1.320 1.323 1.074 1.074 1.073
1.201 1.474 1.338 1.365 1.084 1.085 1.084
1.201 1.474 1.338 1.364 1.084 1.085 1.084
o=C-C LC=C-C LF-C-C LC=C-H7 LC=C-H8 LC-C-H
127.0 123.3 128.1 127.8 121.8 120.0 121.8 123.4 111.3 120.9 112.1 113.1 121.5 121.5 120.8 120.0 121.3 122.0 117.4 114.9 114.1
126.6 122.7 113.0 121.1 121.5 114.2
127.7 123.1 112.5 120.9 121.6 114.6
127.0 123.1 112.4 120.9 121.5 114.6
128.2 128.9 129.4 119.9 119.7 120.8 110.1 110.9 111.2 121.9 121.2 120.7 121.4 117.3 116.5
128.4 121.0 110.2 120.7 121.9 116.9
128.5 119.7 110.8 121.4 120.6 117.5
128.5 119.8 110.6 121.5 120.5 117.5
b
-290.37966 -290.37966
b
-290.37922 -290.37923
b
energy a
-289.52413
See Figure 1 for atom numbering.
1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. a
description v(C-C)str v(C=C)str v(C=O)str v(C-H)str v(C-F)str v(=CHZ)str v(4Hz)str b(C-C=C)bend b(C-C=O)bend p(C-H)rock p( C-F)rock b(4Hz)sciss
b
-289.52373
Not reported.
TABLE 2 Definition and Numbering of Local Symmetry Coordinates of Acryloyl Fluoride no.
1.187 1.464 1.316 1.355 1.071 1.073 1.068
6-31G ref 2
definition" r(C 1-C2) r(C 1=C6) r(C2=05) r(C 1-H4) r(C2-F3) r(C6-H7) r(C6-H8) LC2-C1=C6 L05=C2-C1 (LC2-Cl-H4 -LC6=Cl-H4)/h -L 0 5 4 2 - F 3 ) / f i (LCl-C2-F3 - LCl--C6-H7 (2LH8-C6-H7 LC 1=C6-H8) /fi (LC146-H7 - LCl=C6-H8)/4 (74125 74123 76125 + ~ 6 1 2 3 ) / 2 H4 out of C6-Cl-CZ plane F3 out of Cl-C2=05 plane plane C1 out of H7-C6-H8 (77614 17612 78614 + ~ 8 6 1 2 ) / 2
+
+
+
+
See Figure 1 for atom numbering.
for the u7 and ~ 1 intensities; 5 these will be discussed below). Scale factors were then determined for this conformer with these assignments, using a least-squares optimization procedure, see Table 4. The possible Jacobian ill condition was controlled by the singular value decomposition algorithm.26 The system of linear equations was compatible since only the vibrational frequencies of the trans conformer were involved in the iterative procedure. The computed scale factors are now much closer to unity, ranging from 0.8856 to 0.9999, see Table 4. The scale factors were then transferred to the computed MP2/6-31G*/ /MP2/6-3 1G* force field of s-cis-acryloyl fluoride. The scaled quantum mechanical (SQM) force fields of both conformers are given in Table 5 (presented as supplementary material), and the vibrational frequencies are given in column B in Table 3. Table 3 also contains the calculated values of the IR intensities of all the bands of s-trans- and s-cis-acryloyl fluoride as calculated from the MP2/6-3 lG*//MP2/6-3 1G* force fields.
Discussion The geometrical parameters of the s-trans and s-cis rotamers of acryloyl fluoride as calculated in the present work nearly coincide with the data of ref 2. Furthermore, the calculated changes in the geometrical parameters between the two conformers correspond well with the experimentally measured changes. The
only exception is the length of the formal carbonxarbon single bond. According to experimental measurements,23 this parameter is -0.01 A smaller in the s-cis conformer. However, at all the levels of computation reported in the literature, this bond is predicted to be slightly longer in the s-cis c o n f ~ r m e r . ~It, should ~ be noted that the experimental measurements23 were performed some 25 years ago, which may explain this slight contradiction. The changes in the geometry of acryoyl fluoridedue to rotation about the formal C-C bond are discussed in detail in ref 27 where some unexpected results were noted. It is worthwhile repeating that the C=C-C angle decreases in going from the trans to the cis conformer in acryloyl fluoride, contrary to the behavior of this angle in many diene structures (see, for example, ref 10). It is important to point out that this decrease is found both experimentally and computationally. This suggests that for molecules such as buta- 1,3-diene,IO trans,trans,trans- and trans,cis,tranr-hexa-1,3,5-triene,13 and all-trans-octa- 1,3,5,7-tetraene,I4 where there are no experimental structural parameters of the higher energy (gauche) forms currently available, the computed trend in the C=C-C angle is probably reliable. The "unexpected" behavior of the F-C-C and H-C-C angles in acryloyl fluoride is discussed in ref 27. Note that the values of angles calculated from the moments the 0-C-C and F-C-C of inertia given in ref 24 deviate considerably from those reported in refs 2, 23, and 27. It is rather interesting to note that the SQM force fields for the RHF/6-3 lG//RHF/6-3 lGcalculations, which donot include any polarization functions or correlation effects, and the MP2/ 6-31G*//MP2/6-3 l G * calculations do not differ significantly from each other (see Table 5, presented as supplementary material). The largest changes in the SQM force constants are observed for those with relatively small values. In some cases, , and F13,12, the RHF/6-31G e.g., F3,2, F5.4, F6.4, F8.5, F ~ J F10.3, and MP2/6-31G* force constants differ by more than a factor ~ , FIZ,Z,F12.39 F I Z , ~ , of2, and in other cases, e&, F5,2, F6,h F I O ,Fw, Flz,ll, and F17,16, the signs are different. Interestingly, in some instances, the sign changes occur for both conformers. However, there is only one force constant, Fl2,2, with a significant value, -0.2, which changes its sign in going from the RHF/6-31G/ /RHF/6-31G to the MP2/6-31G*//MP2/6-31GZ level of computation. In general, this suggests that using scaling techniques makes it possible in some cases to obtain a physically meaningful set of force constants using a modest computational level. For s-cis-acryloyl fluoride, the frequencies computed using the S Q M force field and reported in column B of Table 3 are,
1418 The Journal of Physical Chemistry, Vol. 98, No. 5, 1994
De Mar6 et al.
TABLE 3: Experimental and Calculated Vibrational Frequencies of s-traos- and s-cis-Acryloyl Fluoride (cm-l) s-cis-CH24H-CF4 s-trans-CHz=CH-CF=O exptl calcda exptl calcd" sym no. description ref 36 ref4b A IIR B ref 36 ref4b A IIR B A' 1. v(=CHz)a str 3 125vw 3126 3336 1 3140 3120 3330 1 3134 2. v(C-H)str 3057vw 3084c 3273 0 3081 3049 309W 3278 1 3085 3. v(=CHz)s str 3005vw 3058 3236 1 3046 3005vw 3228 1 3038 4. v(C4)str 1833dvs 1803e 1898 224 1803 1826dus 1813e 1898 176 1803 5. v(C=C)str 1636w 163W 1712 8 1633 1634w 1623e 1721 18 1643 6. 6(=CH2)sciss 1412.0m 1413 1487 20 1419 1409.9s 1409 1483 62 1415 7. p(C-H)def 1296.0' 1279' 1341 0 1280 1 2 9 8 . v ~ 1298f 1355 27 1295 8. v(C-C)str 1226s 1225 1292 158 1240 1 1 2 5 . 0 ~ ~ 1125 1188 282 1146 9. p(=CHz)rock 1002.9dm 980 1045 69 1002 (1115.W~) 999 1114 9 1063 10. v(C-F)str 811dm 826c 837 31 807 830m 81W 859 19 826 1 1. p(C-F)def 606.2111 606 608 14 599 495 51oC 497 6 487 12. 6(C-C=O)def 526.0~ 527 539 5 515 626.3~ 627 629 12 608 0 280 268 276 280 2 269 13. G(C-C=C)def 277w 283 29 1 A" 14. X(C-H)wag 999.8dm 990 1035 33 994 990.0111 988 1030 41 994 15. x(=CH2)wag 986.0s 986 995 25 972 977.4111 977 993 14 964 808 27 795 800.2m 805 803 31 792 16. x(C-F)wag 804.6111 800 17. r(4Hz)tw 485.3~ 476 496 0 485 485.3~ 486 502 0 491 0 106 18. r(C-C)tors 115 117 119 1 117 101 108 0 A is for the solution of the vibrational problem with the MP2/6-31G1 force constant matrix. B is for the solution of the vibrational problem with the force constant matrix multiplied by the scale factors given in Table 4. IIR are the infrared intensities (km/mol) as calculated from the MP2/ 6-31G*//MP2/6-31G* force constant matrix. Note that a number of reassignments were made in ref 2. From the Raman spectrum of the liquid. d From the infrared spectrum in an N2 matrix. e From the Raman spectrum of the solid. /Reassigned in this work; u7 s-trans is lower than v7 s-cis. 8 From the infrared spectrum in a neon matrix. TABLE 4: Scale Factors for Force Fields of S - ~ ~ S D Sand - s-cis-Acryloyl Fluoride scale factor scale factor type of coordinate ref 2' this workb type of coordinate ref 1" this workb 1. C-C str 0.8402 0.9170 7. Lo=C-C in-pl 0.7917 0.9168 2. C=C str 0.7573 0.9097 8. L M - F in-pl 0.9021 0.9999 3. C=O str 0.8235 0.8974 9. C-C tors 0.7584 0.9579 . 0.9609 4. C-H str 0.8301 0.8856 10. C-H, =CH2 wag 0.6706 5. C-F str 0.8252 0.9385 11. C = C twist 0.7980 0.8887 0.7944 0.9108 6. LC=C-C, LC=C-H in-pl a RHF/6-31G//RHF/6-31G. Transferred from smaller molecules. MP2/6-31G*//MP2/6-31GS. If an initial scale factor of 0.96 is used for all the coordinates,the least-squaresoptimizationresults in a scale factor greater than unity, 1.0268, for coordinate 11,see Table 2, which could violate the physical meaning of the SQM force field, see ref 8. This is likely the result of a mixing of vibrations u9 and u11 (it should also be noted that ug was measured in a matrix for both the s-cis and s-trans conformers) rather than a result of the inverse anharmonic effect for ~11.29 This problem was solved byfixing the scale factor for the rocking vibration of the C-F bond at 0.9999 and initializingthe remaining scale factors at 0.96 in the least-squares optimization. in general, in good agreement with the latest literature assignments.2 (Note that, as mentioned above, the SQM force field was obtained by transferring the scale factors determined for the s-transconformer to theMP2/6-31G*//MP2/6-31G* force field of s-cis-acryloyl fluoride.) A notable exception is the u9 frequency ( P ( = C H ~ ) ~ & : the computed value, 1063 cm-', lies between the extremevalues accepted or proposed as the experimental frequency in recent papers.2~~ Indeed, in ref 2, assignment of the "strongn band observed by Redington3 at 1115.0 cm-I for acryloyl fluoride in a neon matrix and attributed to vg of the cis conformer was maintained. This value now seems to be overestimated as it is unlikely that a crystal shift could explain entirely a displacement from 1063 to 1115 cm-1 for this frequency. It should be mentioned here that the results reported in ref 3 for this spectral region are somewhat controversial. For example, the number of bands observed was different in argon, neon, and nitrogen mat rice^.^ Perhaps the appearance of the 11 15 cm-I band is connected with a Davydov splitting as was suggested earlier in ref 2. At high dilution in the matrix, the Davydov splitting may disappear and only one band will beobserved. Ifthedilutionisnot sufficient,allthecomponents of the split band may be observed. Indeed, the band at 1 115 cm-l is rather close to the very strong vs band at 1125 cm-I (see Table 3). Also, the corresponding v g band in the case of the s-trans conformer is split.4 Note that the computed intensity for v9(s-
cis) is very low (Table 3). This definitely seems to exclude the strong band at 1125 cm-' and indicates why ug(s-cis) is difficult to assign. In ref 4, infrared spectral bands a t 980 and 999 cm-1 were assigned to u 9 of the s-trans and s-cis conformers, respectively. It seems to us that ug = 999 cm-' is too low for s-cis-acryloyl fluoridecompared to 980 cm-1 for thes-trans conformer. Indeed, increases in the (p(=CH&k) frequency similar to the one obtained here in the calculated spectra, 61 cm-1, have been observed on going from s-trans- to gauche-buta- 1,3-diene"J and s-trans- to s-cis-acrolein.15 Finally, there are some weak bands in the region of 1050-1 100 cm-l in the experimental infrared and Raman spectra of acryloyl fluoride, but apparently, all of them may be assigned as combination bands.3.4 Nevertheless, in earlier work in refs 30 and 3 1, the v 9 vibration of the s-cis rotamer was ascribed to a band at 1068 cm-1. This is in good agreement with our calculated value. However, we must caution this assignment: additional bands at 532, 1077, 1120,452,632, 915, 1141, and 1216 cm-1 were measured in the UV spectrum of acryloyl fluoride3O and treated as fundamentals of the ground electronic state conformers.31 In particular, the first three frequencies listed were assigned to the s-trans form and the remaining bands attributed to the s-cis form. Including the bands a t 1077 and 1120 cm-1 in the list of fundamentals of the s-trans rotamer is based, apparently, on the early assignments suggested by Carlson et al.32 However,
s-trans- and s-cis-Acryloyl Fluoride thevibrational solutions obtained with the MP2/6-3 lG*//MP2/ 6-31G* force fields in the present work strongly suggest that these assignments are erroneous. This thus casts doubt on the validity of the assignment, in those papers, of the 1068-cm-1 band as the u9 mode of the s-cis conformer. Therefore, although the MP2/6-31G*//MP2/6-31G* calculations permit some refinement in the assignment of the v9 vibration for the s-cis conformer of acryloyl fluoride in comparison with the RHF/6-31G//RHF/6-3 1G calculation, the resolution of the assignment of ~ ~ ( s - c irequires s) further experimental and/ or computational investigations. As mentioned by a referee, Raman intensities and depolarization ratios may be most helpful in providing more insight into the molecular force field. Unfortunately, GAUSSIAN90 (and even GAUSSIAN92) do not evaluate them in M P frequency computations. Also, although the Raman spectra of gaseous, liquid, and solid acryloyl fluoride are given in ref 4, the depolarization ratios are not mentioned, and we have not attempted to classify the bands shown in that work according to weak, medium, or strong. The other bands of the s-cis conformer coincide well with the reassignments suggested in ref 2. This is further corroborated by the strong correlation between most of the experimental and theoretical distributions of IR intensities. In this connection, it is interesting to note that the MP2/6-3 1G* intensity calculations maintain the assignmentsof such closely situated bands as 1412.0 cm-1 (s-trans) and 1409.9 cm-l (s-cis): thus, the intensity of the v&-cis) band is computed to be higher than that of the &(strans) bands. However, if one keeps the assignments from ref 3 for ~ ( s - t r a n s and ) ~ ~ ( s - c i s1298 ) , and 1296 cm-I, respectively, then there is disagreement between the experimental and computed IR intensities. In the gas-phase infrared spectrum, the 1298-cm-1 band is much stronger than that at 1296 cm-I (see Figure 2 in ref 3), whereas u7(s-trans) and u7(s-cis) are predicted to be very-weak- and medium-intensity bands, respectively. Note that the frequencies calculated using the unscaled and scaled RHF/6-31G//RHF/6-31G2 and MP2/6-31G*MP2/6-31G* force fields all indicate that q(s-trans) should be lower than q(s-cis). This is contrary to the assignments in refs 2-4 (see Table 3). It is thus suggested that u7(s-trans) and v7(s-cis) be reassigned to 1279 and 1298 cm-l, respectively. As pointed out by a referee, there is also disagreement between the calculated intensity for vls(s-trans), “medium”, and the experimental attribution “strong”. However, examination of the spectra in Figure 2 of ref 3 shows that “medium” would be a better attribution since there is a very strong, broad, underlying absorption in the vapor-phase spectrum. It is worthwhile to mention the change of the frequency values for the p(C-F) and p(C-C=O) vibrations in going from the s-trans to the s-cis forms of this molecule (see Table 3 and ref 2). These frequency shifts (about 100 cm-l) are associated with changes in the signs for some of the off-diagonal elements of the force constant matrix for the s-cis conformer, which connect the 9 and 11 coordinates (see Tables 2 and 5 ) , e.g., F9,2, F9,4, F9,6, F9,7, F I O , Fi1.4, ~ , Fi1.7, Fii,io9F I Z , ~Fiz,ii, , F i v , and F13,ii. Moreover, two elements (F11,7 and Fll,lo) change the order of their magnitudes. The last two force constants are also connected with the corresponding shifts for the u(C-C) and the p(=CH2) vibrations. The same is true for F4.1 and Flz,l. Analogous changes in the sign or the relative magnitude are also observed for some out-of-plane off-diagonal force constants, e.g., F15,14,F16.14,F16.15, F17,14,F17,1~t F18.14, Fls,ls,and F18,17,whichare obviously connected with the change in orientation of some structural units in going from the s-trans to the s-cis rotamer. However, it should be mentioned here that there are another 16 small force constants, which belong to the in-plane portion of the force field, which change either their signs or their relative magnitudes in going from the trans to the cis form. While this may be an indication of the limits on the accuracy of the calculated force constants,
The Journal of Physical Chemistry, Vol. 98, No. 5, 1994 1419 at this computational level it may just show again that a force field must be considered in toto, since there are many correlations among its separate force constants. Finally, besides the shifts of the u ( C - C ) ~ ~ ~P ,( = C H ~ ) ~ =and ~, 6(C-c=O)def bands, which are also characteristic of the parent acrolein molecule,15 acryloyl fluoride exhibits a shift of the p(C-F)dcf band in going from the s-trans to the s-cis rotamer. Conclusions The MP2/6-3 lG* calculations reported above on the s-cis and s-trans conformers of acryloyl fluoride corroborate, in general, with the fundamental frequency reassignments suggested in ref 2 on the basis of the RHF/6-31G calculations. A notable exception is u9 for s-cis-acryloyl fluoride which must now be considered as unknown. Also, the computed MP2/6-3 l G * relative intensities, as well as the frequencies obtained using the unscaled and scaled force fields at both levels of theory, require that the assignments for v7(s-trans) and u7(s-cis) be changed to 1279 and 1298 cm-*, respectively. ComparisonoftheRHF/6-31Gand MP2/6-31G* SQMforce fields shows that a physically meaningful force field can indeed be obtained from moderate level calculations, which do not include polarization functions or take into account electron correlation, using scale factors obtained from related smaller molecules. This will allow ab initio assisted vibrational analysis to be carried out on much larger molecular systems. The MP2/6-31G* force field also reveals that there are many differences in the signs and/or the relative magnitudes of a large number of off-diagonal force constants in the s-trans and s-cis conformers of acryloyl fluoride. This demonstrates the inherent inaccuracy of the traditional approach of transferring experimental-based force constants from one rotational conformer to another. In general, it should be noted that the ideology of the scaling method and the transferability of scale factors are based on the assumption of equal percentage of overestimation of quantum mechanical calculations of the nonscaled force fields (in the case of moderate basis sets) for the identical structural moieties of the related molecules. In turn, the traditional transfer of force constants as calculated by solution of the inverse vibrational problem is based on the additivity principle. However, in the case of vibrational spectroscopy, this principle was shown17 to be a rather approximate approach and the corresponding force fields need to be “corrected” for each new molecule involved in the process of transferability of force constants obtained by solution of the inverse vibrational problem. Acknowledgment. The authors wish to thank George DestrCe of the U.L.B./V.U.B. Computing Center for his assistance with GAUSSIAN90. Yu.N.P. thanks the U.L.B. for an international scientific collaboration grant and the Russian Foundation for Fundamental Investigations for Grant 93-03-1 8386. G.R.D. thanks the Belgian National Foundation for Scientific Research (F.N.R.S.) for a travel grant. Supplementary Material Available: Table 5, scaled quantum mechanical (SQM) force fields of s-trans- and s-cis-acryloyl fluoride (mdyn/A, mdyn, mdynA) as calculated at the RHF/ 6-31G//RHF/6-31G and MP2/6-3 lG*//MP2/6-3 l G * levels of theory (3 pages). Ordering information is given on any current masthead page. References and Notes (,l) On leave from Laboratory of Molecular Spectroscopy, Chair of Physical Chemistry, Department of Chemistry, Moscow State University, 118999, Moscow, Russian Federation, C.I.S. (2) Bock, Ch. W.; Panchenko, Yu. N.; Krasnoshchiokov, S.V. Chem. Phys. 1990, 147, 65-15. (3) Redington, R. L. J . Chem. Phys. 1975, 62, 49274936. (4) Durig, J. R.; Berry, R. J.; Groner, P. J. Chem. Phys. 1987,87,63036322.
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