Oxygen-17-induced proton relaxation rates for alcohols and alcohol solutions

Abstract

O uso de amostras de álcoois enriquecidas com 17 O, para medir o tempo de correlação do vetor internuclear OH funciona bem quando o próton da hidroxila sofre uma troca rápida. Para álcoois tais como metanol e etanol, a taxa de troca da hidroxila para amostras puras é relativamente lenta, mesmo à temperatura ambiente, podendo resultar em erros sistemáticos significativos se efeitos de troca lenta não forem considerados. Para trocas lentas do próton da hidroxila, 17 OH, o sinal é uma função relativamente complexa da razão de troca química do próton da hidroxila, do acoplamento de spin OH (cerca de 80 Hz para álcoois e água) e do tempo de relaxação para o oxigênio. A largura de linha do OH pode tornar-se tão grande, devido à relaxação escalar com o núcleo de oxigênio, que o sinal torna-se muito difícil de detectar. Para etanol puro enriquecido com 17 O, à temperatura ambiente, o tempo de relaxação do oxigênio é de aproximadamente 3,0 ms e a largura de linha do próton da hidroxila é maior do que 1000 Hz.The use of 17 O enriched samples of alcohols to measure the correlation time of the OH internuclear vector works well when the hydroxyl proton exchanges rapidly. For alcohols such as methanol and ethanol the hydroxyl exchange rate for neat samples is relatively slow even at room temperature and significant systematic errors result if slow exchange effects are not considered. For slow exchange the hydroxyl proton, 17 OH, signal is a relatively complex function of the chemical exchange rate of the hydroxyl proton, the OH spin coupling (about 80 Hz for alcohols and water) and the relaxation time for the oxygen. The OH linewidth can become so large due to scalar relaxation with the rapidly relaxing oxygen nucleus that the signal becomes very difficult to detect. For neat 17 O enriched ethanol at room temperature the oxygen relaxation time is about 3.0 ms and the hydroxyl proton linewidth is over 1000 Hz.

Ab initio calculation of 1H, 17O, 27Al and 29Si NMR parameters, vibrational frequencies and bonding energetics in hydrous silica and Na-aluminosilicate glasses

“…Liu et al (2002) suggest that perhaps the size of the aluminosilicate clusters may not be sufficiently large to account for all of the contributions to the C q and that this discrepancy maybe an artifact of the theoretical method. However, there is excellent agreement between calculated and experimentally-derived C q s for all nuclei in simple alcohol and amide complexes (Ludwig et al, 1995;Farrar et al, 1999). Furthermore, from the current work and in Kubicki and Toplis (2002), 27 Al C q values generally seem to be overestimated by Ϸ1 MHz wherever comparisons are unambiguous, significantly less than the predicted change of 10 MHz with protonation of the bridging oxygen.…”