Quantum Mechanical Study on the Structure and Vibrational Spectra of Cyclobutanone and 1,2-Cyclobutanedione

Table 5

Comparison of experimental raman spectra with calculated vibrational wave numbers (cm⁻¹) with % PED contribution of corresponding modes in brackets using different methods and 6-31G (d, p) basis set combination of 1,2-cyclobutanedione under symmetry consideration.

S. no.	Assignments	Exp. (Raman values)	Calculated
			B3LYP	B3PW91	LSDA	MP2
For b2 modes
1	𝜈 as(CH2) (99)	3011	3021	3037	3114	2974
2	𝜈 as(C–O) (90)	1802	1838	1852	1780	1831
3	𝛾(CH) + Sa(CH2) (97)	1382	1390	1380	1431	1312
4	Ring distortion	919	905	923	945	878
5	Ring distortion	713	715	715	726	683
6	Ra(CH2)	287	280	284	287	262
7	Asym.(C–C=O) bending		279	281	282	245
For b1 modes
8	𝜈 s(CH2) (99)	2952	2963	2974	3041	2912
9	𝜔(CH2)		997	994	1006	953
10	𝜔(CH2) + 𝜔(C–C=O)		932	934	955	927
11	𝜔 s(C–C=O)		696	695	712	631
For a1 modes
12	𝜈(CH) (100)	2964	2969	2980	3047	2917
13	𝜈(C=O) (95)	1765	1792	1810	1736	1786
14	Symm.(CH) bending + s(CH2) (97)	1403	1408	1401	1451	1333
15	𝜈(C–C) Ring breathing		1161	1162	1189	1118
16	Ring distortion		1017	1030	1054	1008
17	Rs(CH2)	979	935	941	961	928
18	Ring distortion		700	710	720	673
19	Symm.(C–C=O) bending	487	495	492	489	448
For a2 modes
20	𝜈 as(CH2) (99)		3009	3025	3103	2963
21	Ra(CH2) (88)	1210	1188	1186	1210	1130
22	𝛽(C–H) (87)	1181	1167	1166	1195	1121
23	Asym.(C–C=O) bending + 𝜔(CH2)		517	525	525	497
24	Ring twist		86	82	70	72