Journal of the American Chemical Society, Vol.117, No.30, 7974-7986, 1995
Polarization Assignments in the Vacuum UV Spectra of the Primary Amide, Carboxyl, and Peptide Groups
Polarized VUV absorption spectra of single crystals of propanamide and N-acetylglycine to 83 000 cm(-1) (120 nm) are presented. The absorption curves are obtained from the corresponding reflection spectra by Kramers-Kronig analysis. The NV1 transition of propanamide appears at 53 000 cm(-1) (185 nm) and is polarized at -35 degrees +/- 3 degrees. (The C-O axis is taken as the reference axis, and positive angles are measured toward the N atom of the amide group.) The second pi pi* transition appears near 78 000 cm(-1) (128 nm) and is polarized at +46 degrees +/- 8 degrees. Evidence for a weaker, in-plane polarized band (amide R(1)) neat 51 000 cm(-1) (196 nm) is presented. This band is in addition to the n pi* (amide W band) which appears in solution at lower energy yet, 45 500 cm(-1) (220 nm). The spectrum of N-acetylglycine is basically a superposition of the spectra of a secondary amide (peptide) linkage and a carboxyl group. The peptide NV1 band appears at 52 000 cm(-1) (192 nm) with a polarization direction of -55 degrees +/- 5 degrees. A band that appears near 64 000 cm(-1) (156 nm) and that has no counterpart in the simple amide spectrum is assigned as the carboxyl NV1’ (prime signifying local to the carboxyl) and is polarized locally at either -61 degrees or 4 degrees both +/-3 degrees (C-O reference axis, positive toward hydroxyl). The amide NV2 band appears near 72 000 cm(-1) (139 nm) with a polarization of 10 degrees or 61 degrees both +/-10 degrees. Evidence for other, weaker bands is presented. INDO/S calculations for both systems are found to be in reasonably good agreement with the experimental results. Exciton mixing calculations suggest that crystal interactions do not lead to significant changes in apparent transition moment directions.