Pure Rotational Absorption Spectra

At room temperature, many rotational levels of a molecule are well-populated because of the small energy differences between rotational levels. Thus, in contrast to electronic and vibrational absorption spectra, most of the lines in a pure-rotational absorption spectrum do not involve the ground state. 

Which of the following molecules will exhibit a pure rotational absorption spectrum ... 

Second, due to the nonzero matrix elements of the operator of the dipole moment, transitions between the rotational sublevels of the same vibronic level with the change of the projection K by unity (AA = 1) are possible. In the work of Child and Longuet-Higgins (1961) it is noted that the possible vibronic pure rotational absorption spectrum within the same vibronic level is somewhat similar to the pure rotational spectrum in the excited degenerate dipolar-type state of harmonic oscillators predicted by Mizushima and Venkatesvarlu (1953). 

E13.12(b) Polar molecules show a pure rotational absorption spectrum. Therefore, select the polar molecules based on their well-known structures. Alternatively, determine the point groups of the molecules and use the rule that only molecules belonging to C , C v, and Cs may be polar, and in the case of C and C v, that dipole must lie along the rotation axis. Hence all are polar molecules. 

By definition, molecules that are spherical tops do not have a permanent dipole moment, so they do not have a pure rotational spectrum. However, imder some conditions they may have rotational absorptions superimposed in their vibrational spectrum. 

The overall OD vibrational distribution from the HOD photodissociation resembles that from the D2O photodissociation. Similarly, the OH vibrational distribution from the HOD photodissociation is similar to that from the H2O photodissociation. There are, however, notable differences for the OD products from HOD and D2O, similarly for the OH products from HOD and H2O. It is also clear that rotational temperatures are all quite cold for all OH (OD) products. From the above experimental results, the branching ratio of the H and D product channels from the HOD photodissociation can be estimated, since the mixed sample of H2O and D2O with 1 1 ratio can quickly reach equilibrium with the exact ratios of H2O, HOD and D2O known to be 1 2 1. Because the absorption spectrum of H2O at 157nm is a broadband transition, we can reasonably assume that the absorption cross-sections are the same for the three water isotopomer molecules. It is also quite obvious that the quantum yield of these molecules at 157 nm excitation should be unity since the A1B surface is purely repulsive and is not coupled to any other electronic surfaces. From the above measurement of the H-atom products from the mixed sample, the ratio of the H-atom products from HOD and H2O is determined to be 1.27. If we assume the quantum yield for H2O at 157 is unity, the quantum yield for the H production should be 0.64 (i.e. 1.27 divided by 2) since the HOD concentration is twice that of H2O in the mixed sample. Similarly, from the above measurement of the D-atom product from the mixed sample, we can actually determine the ratio of the D-atom products from HOD and D2O to be 0.52. Using the same assumption that the quantum yield of the D2O photodissociation at 157 nm is unity, the quantum yield of the D-atom production from the HOD photodissociation at 157 nm is determined to be 0.26. Therefore the total quantum yield for the H and D products from HOD is 0.64 + 0.26 = 0.90. This is a little bit smaller ( 10%) than 1 since the total quantum yield of the H and D productions from the HOD photodissociation should be unity because no other dissociation channel is present for the HOD photodissociation other than the H and D atom elimination processes. There are a couple of sources of error, however, in this estimation (a) the assumption that the absorption cross-sections of all three water isotopomers at 157 nm are exactly the same, and (b) the accuracy of the volume mixture in the... 

For UV spectra of parent and substituted 1,2,3-triazoles and benzotriazoles, see CHEC-I <84CHEC-1(5)684 >. The UV spectra of benzotriazole, 1-methyl- and 2-methyl-benzotriazole in the gas phase at 90°C have been recorded <94JOC2799>. 1-Alkyltriazolines show two A ax in acetonitrile, 239-242 and 263-266 nm, both with log e w 3.50 <93JOC2097>. The UV spectra of bicyclic triazolines (754) have been recorded <9lJOC4463>. The Si-So electronic absorption spectrum of 1/f-benzotriazole at 286 nm has been studied by computer simulation of the rotational contours. The result shows that the benzotriazole band is an almost pure type-5 band <93JSP(158)399>. 

Sakai, H. (1962). A Slit Function Correction and an Application to the Study of the Absorption Lines in the H20 Pure Rotation Spectrum. U. S. Armed Services Technical Information Agency Report AD287897. 

H2-X where X is a molecule. If a molecule other than H2 is chosen as the collision al partner X, new absorption bands appear at the rotovi-brational bands of that molecule. As an example, Fig. 3.17 shows the rototranslational enhancement spectra [46] of H2-CH4 for the temperature of 195 K. At the higher frequencies (v > 250 cm-1), these look much like the H2-Ar spectrum of Fig. 3.10 the H2 So(J) lines at 354, 587, and 815 cm-1 are clearly discernible. Besides these H2 rotational lines, a strong low-frequency spectrum is apparent which corresponds to the (unresolved) induced rotational transitions of the CH4 molecule these in turn look like the envelope of the rotational spectra seen in pure methane, Fig. 3.22. This is evident in the decomposition of the spectrum, Fig. 3.17, into its main components [46] the CH4 octopole (dashed curve) and hex-adecapole (dot-dashed curve) components that resemble the CH4-CH4 spectrum of Fig. 3.22, and the H2 quadrupole-induced component (dotted curve) which resembles the H2-Ar spectrum, Fig. 3.14. The superposition (heavy curve) models the measurement (big dots) closely. Similar spectra are known for systems like H2-N2 [58]. 

Figure 3.10 The electronic transitions [absorption in (a)] of small molecules show vibrational and rotational lines in addition to the purely electronic spectrum, (b) Luminescence emission is resonance fluorescence (f), and chemical reactions (R) can originate from several excited states...

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... 

Pure rotational and vibrational Raman spectra of At2 Raman spectroscopic study of kinetics of Ar, formation in a supersonic expansion seeded with Nj Electronic absorption spectrum of HgAr Rotational Zeeman effect in ArHBr t HgCl2 collision complex formed in harpoon reaction of Hg with Clj investigated via excitation of the HgCL van der Waals complex... 

Microwave absorption spectroscopy has been used to investigate OH, OH and OD produced by electric discharges in the appropriate water vapour . The absorptions lie in the region 7.7 to 37 kMc.sec" and 2uise from transitions between the A-doublets, and (the pure rotation spectrum for OH, a light radical, is in the far infrared). Zeeman modulation was used with a conventional microwave spectrometer. 

It is interesting to compare the results obtained in this section for the absorption spectrum in a viscous medium with the pure rotational spec-... 

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