Hydrogen bonding dimer

TTF salts containing similar ferrocenium anions have been prepared through metathesis reactions between (TTF)3(BF4)2 and the tetraphenylphosphonium salts of the desired anions [71]. The crystal structure of (TTF)(CpFeCp-CONHCH2 S03)-1/3H20 contains isolated TTF dimers. Hydrogen bonds of the —NH—OS- and NH—0=C- type are present between anions. Two phases of the TTF salts with the... 

Swem dimer, hydrogen bonding, 104, 105 Synchronous transition states, peracid alkene epoxidation, 50-8 syn selectivity, regioselective aUyUc... 

In addition to frequency shifts, new vibrational bands occasionally appear in the liquid state as compared to the gas phase. An example is formic acid, HCOOH. At high temperatures, the vapor shows a characteristic O—H stretching vibrational band at 3750 cm-1 as the temperature of the gas is lowered, this band disappears and a new band at 3080 cm -1 appears. In the liquid state, only the 3080 cm"1 band is evident. The new band is due to O—H vibration in the hydrogen-bonded dimer. Hydrogen bonding also affects the IR spectra of liquid CH3OH and H20. 

Boyarchuk and Volkenshtein354,355 have studied the IR spectra of the crystals of 5-bromo-l-methyl-, 5-bromo-3-methyluracil, and 1-methylthymine and discussed the effect of the electronegative substituents in the pyrimidine ring on the potential energy curve of dimeric hydrogen-bonded complexes of uracil derivatives. Features of the IR spectrum of 5-bromo-l-methyluracil crystal suggested that the stability of the lactim form 28 increases owing to the influence of the substituents. 

It was observed that needle-like crystals of the metastable Form-II of the carbamazepine-isonicotinamide cocrystal formed when the ingredients were initially dissolved in ethanol, but that over time plate-like crystals of the stable Form-I grew in the suspension [44]. It was concluded that the molecular packing in Form-II consisted of carbamazepine dimers hydrogen-bonded to nicotinamide chains, and where the pyridine nitrogen of isonicotinamide did not participate in the hydrogen bonding. 

Jensen, J. H. and Gordon, M. S., Ab initio localized charge distributions Theory and a detailed analysis of the water dimer-hydrogen bond, J. Rhys. Chem. 99, 8091-8097 (1995). 

J. H. Jensen and M. S. Gordon, Initio Localized Charge Distributions Theory and Detailed Analysis of the Water Dimer-Hydrogen Bond, J. Phys. Chem. 99, 8091-8107 1995. 

The packing of the C2/c polymorph of diclofenac 73 exhibits regular acid dimer hydrogen bonds and rhombic channels along [001] (Fig. 2.1.59), which should endow it with solid-state reactivity. 

Dimer structures containing two O-H - -O (OH/OH dimer) or two O-D- -O (OD/OD dimer) hydrogen bonds were dissolved in CCI4 at concentrations between 0.2 and 0.8 M. Two-color pump-probe experiments with independently tunable pump and probe pulses were performed with a 100 fs time resolution. Approximately 1% of the dimers present in the sample volume were excited by the 1 pj pump pulse. After interaction with the sample, the probe pulses were spectrally dispersed to measure transient vibrational spectra with a spectral resolution of 6 cm 1. 

Scheme 7 Infinite chain of Ag(I)(3-aldoximepyridine)2 cations held together by oxime-. -oxime dimeric hydrogen bonds.

Figure 1.1 Optimized water dimer hydrogen bond angles.

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