H-Bonding Studies

The position of conformational equilibrium in OH-substituted heterocycles may be estimated by IR H-bonding studies.1428 For example, for the equilibrium 95 96 97, two bands at 3609 cm 1 (free OH) and 3475 cm 1 (intramolecular H-bonded OH) are observed. The % trans-fused conformer (60%) was calculated by comparison of the integrated area of the 3609 cm 1 band with that of the model compound 98.143 

A value of AG° for the quinolizidine equilibrium was obtained by IR spectroscopic techniques. Dilute solution measurements (0.006 M CC14) in 10-cm cells on trans-(l-H,9a-H)-l-hydroxyquinolizidine (99 100 101) showed a small band at 3250 cm-1 corresponding to the H-bonded ds-fused con-former 101. The epimeric cis-(l-H,9a-H) compound was taken as a 100% H-bonded model, and the concentration of the cis-fused conformer 101 was then deduced. A correction must be made for the presence of 100, which was estimated by studies on the conformational equilibrium in l-methyl-3- 

Analysis of the microwave spectrum of piperidine and of A-deuteropiper-idine suggests that the strongest Q-branch series together with the associated R-branch lines arise from the N-H-axial conformer.144 From this absorption and from a weaker series of Q branches, /eq//aj (the relative intensities of the type-A lines of /V-Hcq and N-Hax conformers corrected for the frequency factor) was estimated as 1/6 at — 34°C. This ratio is related to AE = En Hax - jv-Hcq by the expression 

Values of /rc and /iceq were obtained from the linear Stark effects, and, making the assumption that the total dipole moment of both conformers is the same, then nf /paeq was estimated. Substitution in the equation gave AE = 245 + 150 cal mol 1 corresponding to 60% N-Heq conformer at 20°C.144 The validity of the assumption of equal overall dipole moments of the two conformers has been challenged.9 

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G. Theory of Hyperfine Coupling to a-Protons The discovery of this effect in aromatic radicals (Lipkin, 1953) was quite unexpected since the unpaired electron has a node in the plane containing the C—H bonds. Studies of deuterated radicals showed that this coupling was not linked to out-of-plane bending vibrations of the C—H bond as had been originally supposed, and McConnell... 

In H bonding studies the amount of solvent effect to be expected in a normal solvent is estimated, and the additional part of A/i is assigned as the H bonding effect. Some of the recent work is by Few and Smith (646), Mecke (1292), and others (469, 1482, 1901). There is unavoidably an overlap between the effect that can be ascribed to nonspecific electrical interaction as with CCh or hexane, and H bonding between solute and solvent. We include benzene in the latter category because other work shows that its interactions fit our definition (see Section 6.4). 

H-bonding study of alcohols at several temperatures. clodextrin ... 

The direction of addition of B—H bonds to olefins is such as to attach the boron generally to the less substituted of the two carbon atoms of the double bond, as the orientation is governed by the polarities of the olefin and of the B —H bond. Studies with cyclic olefins and with acetylenes have shown cis addition of B—H to occur, and a four-centre transition state appears likely ... 

These charge-transfer structures have been studied [4] in terms a very limited number of END trajectories to model vibrational induced electron tiansfer. An electronic 3-21G-1- basis for Li [53] and 3-21G for FI [54] was used. The equilibrium structure has the geometry with a long Li(2)—FI bond (3.45561 a.u.) and a short Li(l)—H bond (3.09017 a.u.). It was first established that only the Li—H bond stietching modes will promote election transfer, and then initial conditions were chosen such that the long bond was stretched and the short bond compressed by the same (%) amount. The small ensemble of six trajectories with 5.6, 10, 13, 15, 18, and 20% initial change in equilibrium bond lengths are sufficient to illustrate the approach. 

Infrared absorption properties of 2-aminothiazole were reported with those of 52 other thiazoles (113). N-Deuterated 2-aminothiazole and 2-amino-4-methylthiazo e were submitted to intensive infrared investigations. All the assignments were performed using gas-phase studies of the shape of the vibration-rotation bands, dichroism, isotopic substitution, and separation of frequencies related to H-bonded and free species (115). With its ten atoms, this compound has 24 fundamental vibrations 18 for the skeleton and 6 for NHo. For the skeleton (Cj symmetry) 13 in-plane vibrations of A symmetry (2v(- h, 26c-h- Irc-N- and 7o)r .cieu.J and... 

Together with pyridones, the tautomerism of pyrazolones has been studied most intensely and serves as a model for other work on tautomerism (76AHC(Sl)l). 1-Substituted pyrazolin-5-ones (78) can exist in three tautomeric forms, classically known as CH (78a), (DH (78b) and NH (78c). In the vapour phase the CH tautomer predominates and in the solid state there is a strongly H-bonded mixture of OH and HN tautomers (Section 4.04.1.3.1). However, most studies of the tautomerism of pyrazolones correspond to the determination of equilibrium constants in solution (see Figure 20). 

Analysis of CEELS line shapes often show chemical shifts that have been used to study FeB alloys after recrystallization, C-H bonding in diamondlike films and multiple oxidation states. 

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