Similar Wavenumbers Research Articles

The role of modal coupling in the radiation of sound from vibrating structures

The acoustic power radiated by a finite beam excited by a harmonic point force was formulated analytically and shown to be composed of two sets of terms: the first describing the power radiated by the individual modes, the so‐called squared terms; and the second, or cross terms, describing the power radiated by the interaction of different modes. The relative contributions of each were examined as a function of the critical frequency and the drive point location. Results indicated that the cross terms were important primarily below acoustic coincidence and could account for either an increase or a decrease in the radiation efficiency, depending upon the location of the excitation point. In addition, modes with similar wavenumbers were found to interact more strongly than those with significantly different wavenumbers. [Work supported by NSF.]

POLARIZED UV‐ABSORPTION SPECTRA OF RETINAL ISOMERS‐I. MEASUREMENTS IN EXTREMELY THIN MONOCRYSTAL PLATELETS

Abstract Crystals of all‐trans retinal and both different forms of 11‐cis, 12‐s‐cis retinal were grown on quartz slides with faces (101), (001) and (101), respectively, forming thin platelets of less than 0.2 μm thickness. Polarized UV absorption spectra at room temperature were measured in the range from 20 to 43 × 103 cm−1 with a microscope‐spectrophotometer. In this spectral range three diffuse absorption bands were observed for all crystal types at similar wave numbers. A main absorption band was found at 25–28 × 103 cm−1, and two further bands at 32–34 and 38–40 × 103 cm−1. In case of all‐trans retinal the latter band is by far the weakest in this spectral range. Additionally, the crystal spectrum of all‐trans retinal shows a shoulder at the low wavenumber side of the main band which cannot be resolved in the corresponding solution spectrum. In the crystal spectra of 11‐cis, 12‐s‐cis retinal, however, only a strong dissymmetry is observed at this side of the main band.

Luminescence and absorption spectra ofM2UO2Cl4·xH2O

Polarized luminescence and electronic absorption spectra have been recorded at temperatures down to 4 K for M 2UO2Cl4·xH2O (M=K, Rb, NH4). The most important difference between these luminescence spectra and that of Cs2UO2Cl4 arises from the contribution of vibrations of water molecules to the vibronic intensity. In the electronic absorption spectra the wavenumbers of these vibrations may be used to locate and identify electronic origins. Ten electronic origins have been identified at similar wavenumbers to those reported by Denning et al. for Cs2UO2Cl4. New ground state vibrational assignments for M 2UO2Cl4·xH2O are made from 85 K infrared and 120 K Raman data.

Absorptions of methyl and t-butyl halogenobenzoates in the infrared carbonyl region

Solutions of the methyl and t-butyl esters of o-, m-, and p-fluoro-, -chloro-, and -bromo-benzoates have been examined in the i.r. CO region. All but one of the o- and p-esters show doublets whereas the m-isomers give single bands. The doublets of the o-esters arise from rotational isomers, the thermochemically less stable syn-s-trans forms absorbing at higher wavenumbers than the anti-s-trans rotamers; those of the p-isomers are caused by the operation of Fermi resonance. It is probable that the rotamers of the m-esters have similar CO wavenumbers. A convenient, safe procedure for preparing t-butyl esters has been developed.

Structure of the NH stretching vibrational band of pyrazole. Multiple resonance of substances forming strong H or D bonds

For self-associated pyrazole and its adducts with various acceptors the sub-bands appearing in the NH stretching vibrational range are found at approximately the same wavenumbers. From this it is concluded that the bands cannot result from coupling of the stretching vibration to the intermolecular vibrations. The appearance of the bands at similar wavenumbers and an intensity rise with increasing overlap of the NH band suggest the interpretation from Fermi interaction of the stretching vibration with overtones and combinations of the lower-frequency intramolecular vibrations. From the two NH vibrations of the self-associated compound at 3260 and 3170 cm −1 only the vibration at 3170 cm −1 is significantly involved in the interaction. To the four most intense sub-bands binary combinations can be assigned in which the ring vibration at 1543 cm −1 is participating. The sub-bands at the lower frequencies follow as combinations in which the NH deformational vibration takes part. The simultaneous appearance of the sub-bands in the self-associated spectra, contrary to the observations for imidazole, is explainable by Fermi interaction of only one or such associated species which do not appreciably differ in their overtones and combinations. The occurrence of all sub-bands of the self-associated spectra in the mixed associated spectra leads to the conclusion that all bands except the CH stretching vibrational ones are due to Fermi interaction. Generally, with the formation of strong H or D bonds the frequency of the stretching vibration is markedly lowered. Simultaneously, width and intensity are enlarged, thus enabling the vibration to interact with many combinations of the lower-frequency intra- molecular vibrations. Hence, multiple resonance obviously too explains the complicated structure of the XH stretching vibrational band of other substances forming strong hydrogen bonds.