Infrared active bond compounds

Figure II-9 illustrates the four normal modes of vibraiion of a tetrahedral XY4 molecule. All four vibrations are Raman active, whereas only P3 and are infrared active. Fundamental frequencies of XH4-type molecules are listed in Table II-60. The trends p, and v > V4 hold for ihe majority of the compounds. The XH stretching frequencies may be lowered whenever the XH4 ions form hydrogen bonds with counterions. In the same family of the periodic table, the XH stretching frequency decreases as the mass of the X atom increases. Shirk and Shrivef noted, however, that the p, frequency and the corresponding force constant show an unusual trend in Group HI A ...

XY5 molecules may be trigonal bipyramidal (D i) or tetragonal pyramidal (C4J. Trigonal bipyramidal have eight normal modes of vibration, six of which (Ai, E and E ) are Raman active and five (Ai and E) are infrared active. Normal coordinate studies have demonstrated that in neutral trigonal bipyramidal MX5 compounds, the equatorial bonds are stronger than axial ones. 

The number of normal frequencies depends on the number of atoms or, more accurately, on the number of parameters of the covalent bonds connecting the atoms in a compound. For an iV-atomic molecule it is 3N—5 for a linear and 3N— 6 for a nonlinear structure. Such a normal frequency is infrared-active, and so can be expected to appear in the IR spectrum, wben it results from a periodic change of a dipole moment existing in the molecule or at least in the particular bond concerned. The intensity of a band is proportional to the square of the transition moment. The vibration of a more polar bond such as C—O, therefore, will have a higher intensity than that of a less polar bond such as C—C. 

The Infrared Region 515 12-4 Molecular Vibrations 516 12-5 IR-Active and IR-lnactive Vibrations 518 12-6 Measurement of the IR Spectrum 519 12-7 Infrared Spectroscopy of Hydrocarbons 522 12-8 Characteristic Absorptions of Alcohols and Amines 527 12-9 Characteristic Absorptions of Carbonyl Compounds 528 12-10 Characteristic Absorptions of C—N Bonds 533 12-11 Simplified Summary of IR Stretching Frequencies 535 12-12 Reading and Interpreting IR Spectra (Solved Problems) 537 12-13 Introduction to Mass Spectrometry 541 12-14 Determination of the Molecular Formula by Mass Spectrometry 545... 

The great avidity of mitochondria from rat brain for 3-qulnuclldlnyl benzllate has been mentioned previously (56). Lareson et al. (192) recorded the Infrared spectra of seven esters of benzllic acid, of ei t esters of 3-qulnuclidlnol, and of atropine and scopolamine and attempted to correlate the relative strengths of the intramolecular hydrogen bond with the threshold doses of the various compounds in producing psychotomimetic effects in dogs. The data of Albanus (42) on the psychotomimetic activities of anticholinergic compounds were used for the comparison. Of the esters of benzllic acid. 

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