FOR a number of years after the acceptance of the collinear structure for diphenyls the problem of their configuration attracted little at tention. When suit able o-substituent s are present, the two benzene rings cannot become coplanar or rotate freely about the 1 : 1'-bond. This can be demonstrated unequivocally by optical resolution of typical compounds. In intermediate cases, the compounds can be optically activated but cannot be resolved. In compounds without 2-substituents and in compounds with small 2-substituents, coplanarity is not prohibited (these compounds cannot even be activated) and it was at one time considered that the stabilising effect of resonance between the two rings [1952] ConJiguration and Conjugation in Diphenyl, etc. Part I . 855 would operate to produce a structure in which at least the fireferred configuration was a coplanar one. This view was supported by the results of X-ray crystal analysis of diphenyl (Dhar, Indian J . Physics, 1932, 7, 43), which showed the molecule in the crystal to be coplanar and the 1 : 1'-bond distance to be 1.48 A, and by calculation of the resonance energy of diphenyl from the heat of combustion, which gave a value of about 9 kcal./g.-mol. in excess of the sum of the resonance energies of the two benzene rings (Pauling and Sherman, J . Chem. Physics, 1933, 1, 606, 679). The significance of the latter result is clearly diminished by the fact that diphenylmethane has been stated (Wheland, " The Theory of Resonance," 1944, p. 69) to have a comparable additional resonance energy.During recent years, however, much rather conflicting evidence relating to diphenyl compounds has accumulated, some of which appears to indicate that even in diphenyl itself the preferred configuration of the isolated molecule is not coplanar. At the same time, a number of workers have suggested that partial loss of coplanarity is not necessarily associated with complete loss of resonance. Related to this concept is the growing body of evidence that substituents in the benzene ring may be forced out of the plane of the ring by other groups present (Klevens and Platt,
Abstract. This paper describes a new gas optical depth parameterisation implemented in the most recent release, version 13, of the radiative transfer model RTTOV (Radiative Transfer for TOVS). RTTOV is a fast, one-dimensional radiative transfer model for simulating top-of-atmosphere visible, infrared and microwave radiances observed by downward-viewing space-borne passive sensors. A key component of the model is the fast parameterisation of absorption by the various gases in the atmosphere. The existing parameterisation in RTTOV has been extended over many years to allow for additional variable gases in RTTOV simulations and to account for solar radiation and better support geostationary sensors by extending the validity to higher zenith angles. However, there are limitations inherent in the current approach which make it difficult to develop it further, for example by adding new variable gases. We describe a new parameterisation that can be applied across the whole spectrum, allows for a wide range of zenith angles in support of solar radiation and geostationary sensors, and for which it will be easier to add new variable gases in support of user requirements. Comparisons against line-by-line radiative transfer simulations, and against observations in the ECMWF operational system yield promising results, suggesting that the new parameterisation generally compares well with the old one in terms of accuracy. Further validation is planned, including testing in operational numerical weather prediction data assimilation systems.
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