HCN Dimer

Figure 21-19. Photoelectron spectra of uracil-H20 dimer anion (top), uracil-HCN dimer anion (middle), and uracil-H2S dimer anion (bottom) recorded with 2.540 eV/photon. (Figure 7 of ref. [44]. Reprinted with permission.)...

In another PARS study (Hopkins et al., 1985) it was shown that photoacoustic Raman spectroscopy is a sensitive technique for obtaining Raman spectra of hydrogen-bonded complexes in the gas phase. PARS spectra of the CN stretching region of HCN as a function of pressure revealed bands which could be assigned to HCN dimers and trimers. 

Table 2.41 Optimized geometries (A) and energetics (kcal/mol) of the HCN dimer. contain energetics not corrected for BSSE. See... 

Somewhat later calculations by Kofranek et al. were able to incorporate correlation into the spectral data of the HCN dimer. The SCF data in Table 3.76 - °° confirm the patterns obtained earlier by Somasundram et al. with a different basis set. The correlated results in the next two columns of the table indicate no major changes in patterns. Curiously enough, the inclusion of correlation reduces all of the red shifts while enlarging all the blue... 

Pacansky, H., The infrared spectrum of a molecular aggregate. The HCN dimer isolated in an argon matrix, J. Phys. Chem. 81, 2240-2243 (1977). 

A final example of aqueous media used in the hydrocyanation of butadiene is provided by Waddan at ICI [23]. In this chemistry, copper nitrate salts in aqueous media (among many others) are used for the oxidative dihydrocyanation of butadiene to dicyanobutenes (Eq. 9). Good conversions of butadiene are reported in nonaqueous media but no examples are actually provided in which water is added as a solvent. Moreover, because of problems with alkene and HCN dimerization and the risk of explosion hazards, these reactions appear to work best when conducted stepwise (i.e., HCN addition to catalyst followed by oxidation followed, in turn, by butadiene addition), leading one to wonder about the productivity of these systems. 

Table 10.2. Calculated Vibrational Frequencies of Linear and T Shaped HCCH-HCN Dimers and Monomers [from BlPck et al. (1989)]...

In addition to the relaxation data obtained from measuring lifetimes, one also would like to obtain information on the disposal of excess energy in the fragments which result from vibrational predissociation of the parent complex. This clearly involves the measurement of state-to-state dissociation rates for these complexes. Results of this type have recently been obtained from measurements of the angular distributions of the photofragments and will be discussed for systems such as HF and HCN dimers. In favorable cases data of this type can provide relative state-to-state rates which can be used as sensitive tests of the emerging theoretical methods. Comparisons with simple statistical theories show that the dissociation dynamics associated with these systems is highly non-statistical. 

Smith, I. W. M., Talbi, D., Herbst, E. (2001). The production of HCN dimer and more complex oligomers in dense interstellar clouds. Astronomy Astrophysics, 369, 611. 

---