Some remarks on the assignment of the vibrational spectra of cyclobutane

Method for Evaluating Vibrational Mode Assignments in Surface-Bound Cyclic Hydrocarbons Using Sum-Frequency Generation

“…In contrast to substituted cyclobutanes, the vibrational modes of cyclobutane itself are well characterized and the assignments are consistent. − Assuming that the presence of a substituent does not change the relative frequency order of symmetric and asymmetric stretching motions, peaks occurring above 2940 cm –1 correspond to asymmetric stretching motions, while those at frequencies between 2910 and 2940 cm –1 correspond to Fermi resonances and overtones, and those below 2910 cm –1 correspond to symmetric stretches. − Of course, a comparison to the vibrational spectrum of cyclobutane does not lead to specific assignment of the β- or γ-CH 2 asymmetric stretching motions, as the various CH 2 groups are not distinguished in cyclobutane. ( N.B.…”