J. Phys. Chem. 1984, 88, 4723-4128 DMPO also exhibited much smaller aN and a? values than those of the methyl or carboxy adducts. This variation in the magnitude of the coupling constants in OH-DMPO has also been found in adducts with heteroatoms attached to C2, such as H02-DMP033 (XII) or n-b~toxy-DMP0’~(XIII), which exhibited values of 1.25 11.7G
6.836
aFH, = 2.06G (HI
a y = 1.25G (HI C HZ
xu
m
and 2.06 G, respectively, for one of the methylene protons as well as small a,B values. If a unique proton with a large coupling constant were present at C3, then the W-plan rule3s dictates the occurrence of a pucker at C2 structure (XIV) and the extent of L
5
xm (38) In free radicals, long-range coupling exhibited by a proton separated from the unpaired electron by at least three bonds in a zigzag or W-plan arrangement was first observed by G. A. Russell and K. Y . Chang (Russell, G. A.; Chang, K. Y .J. Am. Chem. Soc. 1965,87,4381. Barfield, M. J. Chem.
Phys. 1964,41, 3825).
4723
ring deformation is expected. to become increasingly pronounced for the adducts of OH, HO,, and O(CH2),CH3 in the given sequence. In general, ring protons with anomalous splittings (e.g., aHY = 3.3 G for 4-phenyl-2,5,5-trimethylpyrrolidine-l-oxyl)14 probably indicate that greater stability is reached through less symmetrical conformations.
Conclusion The present study indicates that with out-of-phase EPR it is possible to obtain greater spectral resolution than with in-phase EPR. This approach allows radicals to be further characterized and permits the identification of species that are not readily distinguishable. We found by computer simulation that with DMPO-dll, the CH3 adduct clearly shows the secondary quartet due to the three equivalent methyl protons. Thus, the use of deuterated DMPO could be a convenient approach for rapidly identifying unknown radicals. Preliminary work indicates that SHF structure can also be observed for adducts of other nitrone and nitroso spin traps.39 Applications could be extended to larger or bicyclic ring systems and measurements could be carried out as a function of temperature and in different solvents. Finally, out-of-phase EPR may prove to be. a promising probe for structural analysis. Registry NO.111,91410-70-7;VI, 40936-05-8;VII, 91410-71-8;VIII, 55482-03-6; sodium formate, 141-53-7; 2-deuteroxyl-5,5-dimethylpyrrolidine-1-oxyl, 80402-64-8; Tempol, 2226-96-2; Tempamine, 14691-88-4;Tempo, 2564-83-2;Ternpone, 2896-70-0;DMPO, 3317-611; DMSO-& 2206-27-1; MezSO, 67-68-5; HZO2,7722-84-1; HzO, 7732-18-5;DZO, 7789-20-0. (39) For a review on the properties of spin traps, see for example: Aurich, H. G. “Supplement F The Chemistry of Amino, Nitroso and Nitro Compounds and Their Derivatives“; Patai, S., Ed.; Wiley: New York, 1982; Part 1, p 565.
A Vibrational Force Field for Acetaldehyde Kenneth B. Wiberg,* Valerie Walters, and Steven D. Colson Department of Chemistry, Yale University, New Haven, Connecticut 06517 (Received: January 17, 1984)
Vibrational force fields were calculated for acetaldehyde with both the 4-31G and 6-31G* basis sets. The scaled frequencies derived from the force fields were in good agreement with the experimental frequencies (rms error 19-27 cm-I). A constrained adjustment of the calculated force constants led to an improved force field which fit the experimental data with an rms error of 9 crn-’. The polar tensors derived from the two basis sets are compared. Dipole moment derivatives also were obtained from the calculations and agree with the qualitative observations on relative band intensities.
Introduction In connection with our studies of intramolecular vibrational relaxation in carbonyl compounds,’ we wished to have a vibrational force field for acetaldehyde. The vibrational spectra of acetaldehyde and its deuterium-labeled derivatives have been reThe observations of Hollenstein and Gunthard (HG)7 were made at a high enough resolution (0.4 cm-’) so that a meaningful comparison of calculated and observed band contours could be made, and their assignment appears to be generally satisfactory. Our examination of the spectra at higher resolution ~~~
~
(1) S . D. Colson, V. Walters, D. Snavely, and K. B. Wiberg, unpublished results. (2) J. C. Morris, J. Chem. Phys., 11, 230 (1943). (3) H. W. Thompson and G. P. Harris, Trans. Faraday Soc., 38, 37 (1942). (4) K. S. Pitzer and W. Weltner, J . Am. Chem. SOC.,71, 2842 (1949). (5) J. C. Evans and H. J. Bernstein, Can. J. Chem., 34, 1083 (1956). ( 6 ) E. F. Worden, Jr. US.A.E.C., UCRL-8508 (1958). (7) (a) H. Hollenstein and Hs. H. Gunthard, Spectrochim. Acta, Part A , 27A, 2027 (1971); (b) H. Hollenstein and F. Winther, J. Mol. Specrrosc., 71, 118 (1978); (c) H. Hollenstein, Mol. Phys., 39, 1013 (1980).
(0.06 crn-’) confirms their assignment for the more intense bands. They also have presented the results of a normal-coordinate analysis. However, it is unlikely that one could derive the correct force field for a molecule of such low symmetry using only experimental data. Thus, we have carried out a calculation of the force field via an ab initio molecular orbital procedure.* The calculated force field was adjusted to give a “best fit” to the observed anharmonic frequencies. The intensities of the bands also have been calculated and are in qualitative agreement with the relative intensities estimated from the reported spectra. It seems appropriate at this time to be specific as to what we imply by an “assignment” of an infrared absorption band. A theoretical calculation will be used to predict a set of harmonic normal modes of vibration. Each normal mode will be characterized by a set of atomic displacement vectors which are best described mathematically in terms of a well-defined set of symmetry coordinates. An observed infrared absorption is then ~~~~~~
(8) P. Pulay in “The Force Concept in Chemistry”, B. Deb, Nostrand, New York, 1981.
0022-3654/84/2088-4723$01.50/00 1984 American Chemical Society
Ed., Van
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The Journal of Physical C h e m i s t r y , Vol. 88, No. 20, 1984
x
Y7
Wiberg et al. TABLE I: Results of Molecular Orbital Calculations H2
h-c H
