General and physical chemistry

Abstract

GENERAL AND PHYSICAL CHEMISTRY.for higher vibrational states. Linnett lo has discussed the internuclear distances and force constants of a number of diatomic molecules in terms of the number of electrons in the bonding region and the 0 or x character of the electrons. There are still few accurate vibrational data for isotopically substituted molecules other than for deuterium substitution. Such data are necessary to evaluate the force constants of the most general quadratic potential-energy function. Thus, for bent XY, molecules, while three fundamental frequencies can be observed there are two primary and two interaction force constants to be evaluated. Smith and Linnett l1 have discussed the relation between these when the three frequencies are known, for a number of molecules of this type, and have indicated ways in which, in the absence of isotopic data, one of the force constants can be estimated, so fixing approximately the values of the others. Besides providing more reliable values of the primary constants the interaction constants provide results of direct chemical interest as they may often be related to electronic changes in one bond caused by stretching or bending another. Smith and Linnett l1 suggest that the compression of the lone pairs may contribute to the bending constant in such molecules as 0,, NO,-, etc. Simpson l2 has pointed out that potential-energy terms linear in angle may occur for the out-of-plane vibrations of planar molecules and that such terms are likely to be important where interatomic repulsion is large.Vibration-Rotation Spectra.-Numerous vibration-rotation spectral studies have been published. Molecules studied include NO l3 (re = 1.1506 A), CS, l4 (CS r, 1.553 A), N,O l5 (the four constants in the quadratic potential-energy expression obtained), HCN and DCN 16-l7 (complete set of vibrational anharmonic constants calcdated),17 0, 18* l9 (with a determination l9 of the four quadratic potential-energy constants from combined use of infrared data and measurements by microwave spectroscopy of centrifugal distortion effects), D,O and HDO 2o (in great detail), H2S 21 (re-assignment of strong absorption round 2600 cm.-l to vl), H2Te,22 CD,F 23* 24 (compared with that in methane, the C-H bond is lengthened and the HCH angle enlarged 23) , C10,F 25 (symmetric top), allene,26* 27* 28 and deuterated allenes 27* 28 (infrared 26, 28 and Raman 27 studies).

Raman Spectrometry

“…Recent papers concerning applications of Raman spectrometry to the determination of molecular structures of specific compounds and their mixtures as well as studies of molecular interactions are also reviewed below. In an effort to limit the length of these sections, we have not, in general, cited papers included in recent reviews by Pullin (215) and Wood'ward (327), both of which are readily accessible to most readers of this review.…”