Microwave and high resolution infrared spectra of vinyl chloride, ab initio anharmonic force field and equilibrium structure

Ab initio anharmonic force field and equilibrium structure of vinyl bromide

“…The agreement between the calculated and the experimental vibrational frequencies m i is rather good, the median of absolute deviations being only 13 cm À1 (1.25%). This is comparable, even slightly better, to the results found for vinyl chloride [17]. Likewise, the deviations are mainly systematic and the largest ones are for the CH bond stretches.…”

“…However, Table 5 shows that the r s -fit model deserves no preference over the r m models. Table 6 makes evident the same, yet unexplained feature which had already been observed in the earlier study on vinyl chloride [17]: the (negative) principal-axis-coordinate a(H c ) is in all experimental determinations by up to 0.02 Å larger than the ab initio estimate (and the semi-experimental equilibrium value). Both, a(H c ) and b(H c ) are large coordinates, the small-coordinate argument for lacking accuracy is hence not applicable.…”

“…Thus, this comparison is not very meaningful, it is nevertheless made in Table 3. What can be said is that the results are of comparable quality to those found for vinyl chloride [17]. Actually, the equilibrium inertial defect is a much better indicator of the quality of the cubic force field.…”

“…For the C@C bond length, the value of the offset was derived from ethylene and vinyl chloride [17], its value is 0.0029 Å . This value should be reliable too because the C@C bond length remains fairly constant.…”

“…It is then possible to calculate an equilibrium structure called semi-experimental structure [15,16]. Finally, as shown in the case of vinyl chloride [17], the mass-dependent r m methods [18] can deliver rather accurate results provided that the number and variety of isotopologues is large enough, which seems to be the case for vinyl bromide. In conclusion, three independent methods may be used to determine the equilibrium structure of vinyl bromide.…”