Effects of {sup 1}H-{sup 1}H spin exchange in the {sup 29}Si CP-MAS NMR spectra of the silica surface
- Colorado State Univ., Fort Collins (United States)
Four {sup 29}Si CP-MAS NMR pulse sequences have been used to study the effects of {sup 1}H spin exchange among various {sup 1}H spin reservoirs on a silica gel surface. Both {sup 1}H-{sup 29}Si dipolar-dephasing experiments and proton-coupled {sup 29}Si CP-MAS results indicate that various hydroxyl groups of silanols in silica gel undergo rapid {sup 1}H spin exchange and that the most strongly coupled protons provide the dominant source of cross polarization to geminal-silanol silicons. {sup 1}H-{sup 1}H dipolar dephasing prior to {sup 1}H {r_arrow} {sup 29}Si cross polarization shows a rapid {sup 1}H spin exchange between {sup 1}H reservoirs of single-silanol groups and of geminal-eilanol groups; however, the {sup 1}H spin exchange rate is too fast to be measured by this strategy. A slower {sup 1}H-{sup 1}H dipolar-dephasing decay due to {sup 1}H spin exchanges is found for the {sup 1}H spin reservoir of single silanol groups. A pulse sequence that utilizes a {sup 1}H-{sup 1}H dipolar-dephasing period followed by an evolution period for z-axis spin exchange prior to {sup 1}H{r_arrow}{sup 29}Si cross polarization provides a measure of a proton (chemical) exchange rate. The results are discussed in terms of {beta}-cristobalite models of the silica surface. 43 refs., 11 figs.
- Sponsoring Organization:
- USDOE
- OSTI ID:
- 411980
- Journal Information:
- Journal of Physical Chemistry, Journal Name: Journal of Physical Chemistry Journal Issue: 10 Vol. 96; ISSN JPCHAX; ISSN 0022-3654
- Country of Publication:
- United States
- Language:
- English
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