Halogen force constants

Infrared spectroscopy has broad appHcations for sensitive molecular speciation. Infrared frequencies depend on the masses of the atoms iavolved ia the various vibrational motions, and on the force constants and geometry of the bonds connecting them band shapes are determined by the rotational stmcture and hence by the molecular symmetry and moments of iaertia. The rovibrational spectmm of a gas thus provides direct molecular stmctural information, resulting ia very high specificity. The vibrational spectmm of any molecule is unique, except for those of optical isomers. Every molecule, except homonuclear diatomics such as O2, N2, and the halogens, has at least one vibrational absorption ia the iafrared. Several texts treat iafrared iastmmentation and techniques (22,36—38) and thek appHcations (39—42). 

Fig. 19 Variation of the intermolecular stretching force constant ka with nucleophilicity Nb for several series of halogen-bonded complexes B- XY, where B is one of a series of Lewis bases and XY is Cl2, Br2, BrCl, C1F or IC1. N% were assigned by use of Eq. 4 with the choice of ICi = 10.0, hence the perfect straight line for the B- -IC1 series. The lines for the other series are those obtained by least-squares fits to the ka values using the Mb determined from the B- IC1 series. Points for E P- -IC1, E N- -C1F and EEN- BrCl are anomalous and were excluded from the fits (see text for discussion)... 

The second well-known electrophilicity or nucleophilicity scale was by Legon and Millen [13,14]. In this scale, the assigned intrinsic nucleophilicity is derived from the intermolecular stretching force constant k, recorded from the rotational and infrared (IR) spectra of the dimer B. .. HX formed by the nucleophile B and a series of HX species (for X halogens) and other neutral electrophiles. The nucleophilicity number in this case is obtained from the empirical relation... 

The frequency variation occurring on going from one molecule to another or to the molecular complex depends on force constants in noncomplexed molecules and reflects, in the final analysis, the ionic nature of the bonds. When no spectra can be obtained in solution or in the gas phase, the inter-molecular contributions should be taken into account, especially in interpreting the Sn—X patterns. It should also be remembered that Sn—X frequencies are markedly dependent on the mass of the halogen involved and increase with atomic number. Nevertheless, the frequencies, which are a function of the nature and the number of halogens and vary over a wide range, are a good reflection of the bond ionicities and of the coordination number. 

The second one with an approximately 60 kcal higher energy. The C—Cl bond has a certain double-bond character of about 15 % this fact and the changed charge distribution—the chlorine more positive than normal—explain the above-mentioned experimental facts. The diminished double-bond character of the C=C bond appears clearly from the experimental data, namely a diminished tendency to addition of halogen, etc., and a diminished force constant, evident from the lowered C=C frequency. 

Vibrational Frequencies and Force Constants for the Triatomic Halogen Cations... 

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