Absorption spectra near-infrared, vibrational-rotational

Most atmospheric visible and DV absorption and emission involves energy transitions of the outer electron shell of the atoms and molecules involved. The infrared spectrum of radiation from these atmospheric constituents is dominated by energy mechanisms associated with the vibration of molecules. The mid-infrared region is rich with molecular fundamental vibration-rotation bands. Many of the overtones of these bands occur in the near infrared. Pure rotation spectra are more often seen in the far infrared. Most polyatomic species found in the atmosphere exhibit strong vibration-rotation bands in the 1 - 25 yin region of the spectrum, which is the region of interest in this paper. The richness of the region for gas analysis... 

A harmonic oscillator can only change its vibrational quantum number by one when it absorbs a photon (Av = 1) therefore, the only frequencies which can be absorbed are near the classical vibrational frequency co = -JkJJi. The absorption will also change the rotational quantum number (Ay = 1). In practice, this means that the infrared spectrum of a small molecule has rotational structure, which permits bond length measurement as well as force constant measurement (Figure 8.6). 

The y-dependence in the rotational constant B is clearly visible in the HCl near-infrared spectrum shown in Fig. 3.3. This vibration-rotation spectrum consists of the v = 0 to v = 1 absorptive transition with rotational fine structure in a P branch (whose absorption lines at successive J values appear at ever lower frequencies according to Eq. 3.64) and an R branch (whose absorption lines run to higher frequencies as J is increased, Eq. 3.65). No Q branch line occurs, because HCl in its closed-shell electronic ground state has no electronic angular momentum. The absorption lines are labeled according to the value of J" in the lower vibrational state R(0) is the R-branch line from J" = 0, P(l) is the P-branch line from J" = 1, etc. The rotational line spacings decrease at higher frequencies in both branches, in consequence of the quadratic... 

Ethylene 1 displays a single sharp line in its p.m.r. spectrum (S = 2-32 p.p.m., external TMS), indicating that rotation about the central C—bonds is rapid on the n.m.r, time scale. The requirement of a center of symmetry imposed by the lack of C=C stretching absorption in the infrared spectrum of 1 (strong Raman band at 1630 cm ) ° is, of course, consistent with a propeller-like structure as well as a planar one, and the steric barrier to coplanarity is considerable. Bond-order calculations, from Hiickel theory and vibrational spectra, indicate that nearly all of the twist in a non-planar structure would involve the C—N bonds rather than the G—G bond. ... 

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