J . Phys. Chem. 1990, 94. 5402-5405
5402
-
x 0
( Z = 3) has a = 8.83 8, and c = 29.96 A. The structure can be described as closest-packed sodium cryptate cations with sodium anions occupying the pseudooctahedral holes. The sodium anions form parallel planes that are perpendicular to the 3-fold axis. Also, both Na+ and Na- have local 3-fold symmetry axes, which requires that they have axially symmetric tensors. This means that one of the principal axes (the principal Z axis) is along the 3-fold axis of the crystal. In summary, this study of the orientation dependence of the NMR spectrum of Na+C222.Na- has yielded accurate values of the quadrupole coupling constants and chemical shifts for both Na+ and Na-. Only an upper limit for the QCC of Na- had been obtained from powder NMR data previously. The results agree with previously determined values from powder spectra and are in accord with the known crystal structure. They confirm the previous assumption that Na- is nearly "gaslike" in the crystalline environment, with, however, some broadening of the NMR spectra of Na- by quadrupolar interactions. Yet, more important than the additional data for NaT222-Na- is the prospect the study provides for single-crystal NMR studies of other alkalides. The NMR sensitivities of Li, Na, and Cs are high enough to make single-crystal NMR studies feasible for crystals as small as about 2 mm on an edge. Such studies can yield both the quadrupolar and chemical shift tensors and their temperature dependence. Since the quadrupolar tensor is very sensitive to the surroundings, phase transitions and orientation-dependent motions can be studied. The present study shows that it is possible to grow suitable crystals of alkalides and measure their NMR spectra in spite of their reactivity and thermal instability.
Calculated Observed
1 R
-801
0
'
'
30
'
'
60'
'
90' ' l h
'1iO'
do
Angle of Rotation (Degree)
1000
-.-P -500j 0
E
LL
-looo-j
-0 5 1-
0
30
60
90
120
150
180
Angle of Rotation (Degree)
Figure 4. Angular dependence of the NMR parameters for a single crystal of Na'C222-Na- at vL = 47.61 MHz and t = -36 "C. (a) Chemical shift variation of the central transition of Na+. (b) Half the separation between the satellites of Na-.
Acknowledgment. This research was supported by National Science Foundation Grant DMR 87-14751 and the MSU Center for Fundamental Materials Research.
the NMR peaks for Na+ are broadened at these orientations. The small quadrupole coupling constant of the sodium anion implies that there is little perturbation of the electron cloud of Na- by surrounding protons and charges. This compound is rhombohedral, with space group R32.22 The hexagonal unit cell
Supplementary Material Available: Derivation of the tensor components, A"& ( 3 pages). Ordering information is given on any curent masthead page.
Raman Studies of the Hydrolysis of Tetramethyl Orthosllicate. 1. Influence of Formamide on the pH M. S. Bradley,* J. H. Krech, and J. Maurer Department of Chemistry, The University of Connecticut, U-60,21 5 Glenbrook Road, Stows, Connecticut 06269-3060 (Received: August 8, 1989; In Final Form: February 26, 1990)
Raman spectra of tetramethyl orthosilicate (TMOS) solutions undergoing hydrolysis are presented. Signals near 830 cm-', previously assigned to a Si-0 vibration, faded as the water:TMOS mole ratio increased from 2:l to 1O:l. A signal at 795 cm-' was observed at low pH values, which we assigned to Next, the simultaneous evolution of the pH and the Raman spectra of TMOS solutions, with and without formamide, were examined. The solutions with formamide showed a pH change, consistently reaching a final pH near 4.7 when the initial pH was between 2 and 5.5. The pH variation, caused by the hydrolysis of formamide, is the primary factor involved in influencing the TMOS hydrolysis.
Introduction The first step in the production of silica glasses by the sol-gel method involves the hydrolysis of an alkoxy silicate, such as tetramethyl orthosilicate (TMOS), in an aqueous medium.14 The silica then polymerizes, producing a suspension of colloidal particles ( I ) Her, R. K. The Chemistry o f s i l i c a ; Wilcy: New York, 1979. (2) Brinker, C. J.; Clark, D. E.;Ulrich, D. R. Better Ceramics Through Chemistry; Materials Research Society Symposia Proceedings 32; Materials Research Society: Pittsburgh, PA, 1984. (3) Artaki, 1.; Bradley, M. S.;Zcrda, T. W.; Jones, J. 1.Phys. Chem. 1985, 89, 4399. (4) Assink, R. A,; Kay, B. D. J . Non-Cryst. Solids 1988, 107, 35.
0022-3654/90/2094-5402$02.50/0
TABLE I: List of TMOS Hydrolysis Products and Their Vibrational Frequencies, following References 3 and 12 silica sDecies Raman frea. cm-I
Si(OCH3)4 Si(OCH3)30H S~(OCH~)~OH)I Si(OCH,)(OH), Si-0-Si (dimers)
644 674 696 726
795
(a sol) and, eventually, a monolithic silica gel. TMOS and water are not miscible, so a cosolvent, usually methanol, is used. Additional solvents, such as drying control chemical additives (DCCA'S),~-' may also be present. DCCA's, particularly form0 1990 American Chemical Society
The Journal of Physical Chemistry, Vol. 94, No. 13, 1990 5403
Hydrolysis of Tetramethyl Orthosilicate and dimethylformamide,'O,'l are used to retard cracking of the gel during drying. When mixed with water, TMOS reacts to produce a range of silica specie^,^-'^-'^ shown in eqs 1-3. hydrolysis: Si(OCH,),(OH),
-
+ XH20 (H3
Si(OCH3),,(OH),+, condensation:
+ HO-Sit S i - O H + HO-Si
