Rings out-of-plane deformations

Macrocycle deformations are observed between 640 and 120 cm in NPcVO and in the range 510-350 cm in Pc VO the highest frequencies correspond to the ring deformations of benzene and pyrrole, while the lowest are macrocycle ring deformations. Bands at 570 and 457cm in PcVO and the band at 537 cm of NPcVO are characteristic of each macrocycle. Bands at 372, 308 and 235 cm observed only in the vanadyl compounds correspond to VO-N vibrations. Other bands in this region are mainly due to macrocycle ring out of plane deformations. 

The prominent IR peaks for 1,2,4-thiadiazoles were attributed as follows to ring skeletal vibrations (1560-1590, 1490-1550 cm-1), to ring breathing and CH-in-plane deformations (1215-1270, 1080-1185, 1020-1050 cm-1), and to CH out-of-plane deformations ( 735 and 795-860 cm-1) <1982AHC285>. 

Ultraviolet spectra of aromatic systems are often used to probe strain-induced perturbations in the K-system. Out-of-plane deformations of the benzene ring shift the 260 nm band to the red and increase its intensity. Classical examples are [2.2]paracyclophane (286 nm) and Pascal s twisted benzenes. The for a given transition reveals changes in the energy of the filled/unfilled gap, whereas the extinction coefficient reveals the efficiency of the transition. 

IR spectral data for most known 1,2,4-triazines have been published. The absorption of the 1,2,4-triazines in the IR region are those expected for this system. The IR spectrum of the parent compound (1) shows three absorption bands for the C—H stretching vibrations at 3090, 3060 and 3030 cm-1, five bands for C=N and C=C stretching vibrations at 1560, 1529,1435,1380 and 1295 cm-1, three for the C—H in-plane deformations at 1163,1135 and 1113 cm-1, two for the characteristic ring skeleton vibrations at 1050 and 995 cm-1 and three bands for the C—H out-of-plane deformation vibrations at 851,768 and 713 cm-1 (68CB3952). These values are in good agreement with similar bands for pyridine, pyridazine, pyrimidine and pyrazine. Alkyl and aryl derivatives of 1,2,4-triazine show similar bands in their IR spectra, with additional bands from the substituents. 

The compounds considered to be thiatriazoline-5-thiones were later shown in fact to be 5-substituted thiatriazoles (Section V). Therefore the assignments by Rao et al.1 of ring breathing and CH out-of-plane deformations to the supposed two different systems of 4- and 5-substituted thiatriazoles all correspond to 5-substituted thiatriazoles.11 The ring breathing absorptions are found at 1320-1260, 1238-1215,... 

V22 CH out-of-plane deformation vi Ring in-plane deformation vi CH out-of-plane deformation 1-10 Ring in-plane deformation VS—V22... 

The plots of the intensities of selected characteristic bands as a function of lateral position (so-called chemical maps) provide information on the amount of the respective molecules or molecular groups in the different morphological structures (Fig. 4.2). The band at 784 cm 1 can be assigned to out-of-plane deformation vibrational modes of the nucleobases cytosine, thymine and uracil and serves as an indicator for the presence of nucleic acids. At 483 cm-1, a C-C-C deformation of carbohydrate polymers such as starch or pectin is present in some of the spectra. To study the distribution of protein compounds, we analysed characteristic signals of the amino acid phenylalanine (1002 cm 1 ring breathe) as well as of the protein amide I band (1651 cm-1) that is brought about by vibrations of the protein backbones. The maximum of the phenylalanine signal co-localizes with a maximum in protein content... 

C-H out of plane deformation showing 1,2-disubstituted benzene ring. 

Fig. 21.3 Probability density function for different vibrational levels of normal vibrations. Dependence of energy change on the value of torsion angle corresponding to the out-of-plane deformation of pyrimidine ring in uracil. MP2/6-31G(d,p) level calculations...

The IR spectra of a number of five-membered N and N,S-heterocyclic compounds have been compared (64CJC43), and the regions of absorption are very similar for the thiadiazole isomers. Characteristic vibrations for 1,2,3-thiadiazoles are 1560-1475, 1350-1280 cm-1 (ring skeletal) 1265-1200, 1190-1175, 1150-950 cm-1 (ring breathing and CH in-plane deformations) and 910-890, 705-670 cm-1 (CH out-of-plane deformations). 

In an oversimplified picture, nonradiative decay in U and C is controlled by a torsional motion about the C(5)C(6) double bond, while in the canonical G tautomer out-of-plane deformations of the six-membered ring are chiefly responsible for internal conversion. In the case of G, the canonical, biologically relevant, 9H-keto form indeed exhibits photophysical properties which are distinctly different from other tautomers. Its excited state lifetime, for example, is the shortest of all tautomers. This is a consequence of its pronounced out-of-plane distortions absent in other tautomers. 

Irrespective of the exact nature of the biexponential fluorescence decay of PdG (emission from two different conformers or bifurcation of the initial irir -state population to two nonradiative decay channels), it is important to note that the subpicosecond excited-state decay, characteristic of guanine or guanosine, is clearly absent in PdG. Thus, the presence of the exocyclic ring, which hinders the out-of-plane deformation of the six-membered ring (C2 in particular), leads to a dramatically reduced internal conversion rate. 

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