3248
J. Phys. Chem. 1984,88, 3248-3253
hydrophobic i n t e r a c t i o n ~ . ~ , A ~ ~associates * ~ ~ - ~ ~also in methanol, although the dimerization constant is expected to be three orders of magnitude lower than in water; in fact, the acridine orange (AO) dimerization constant in methanol was found to be K12e 6.3 Also the adsorption of A on PSS is very strong in water. The evidence that the binding sites correspond to the ionized SO3groups suggests that the main driving force of the binding process is electrostatic; however, other effects must contribute to the process. In fact, the large dye ion is much more strongly bound than small monovalent ions like Na+ bearing a higher charge density. So, the A 0 release from the PSS matrix to water, arising from the Na+ ions competition, was observed only at a salt molar concentration of 0.08-0.10.4 It is reasonable to assume that hydrophobic interactions play a relevant role also in the dye binding process, favoring the displacement of dye molecules from the bulk aqueous solution to the polyelectrolyte phase. The binding strength of A to PSS in methanol is much lower than in water in spite of the stronger electrostatic interactions due (14) Bradley, D. F.;Lifson, S . In “Molecular Association in Biology”; Pullman, B., Ed.; Academic Press: New York, 1968; pp 261-70. (15) Haugen, G. R.; Hardwick, E. R. J . Phys. Chem. 1963,67, 725-31. (16) Uedaira, Ha.; Uedaira, Hi. Kolloid 2.1964, 194, 148-50. (17) Blandamer, M. J.; Brivati, J. A,; Fox, M. F.; Symons, M. C. R.;
Verma, G. S. P. Trans. Faraday SOC.1967, 63, 1850-7. (18) Mukerjee, P.; Ghosh, A. K. J . Am. Chem. SOC.1970,92,6419-24.
to the decrease of the medium dielectric constant. Furthermore, the competition of small ions such as Na+ and H+(Figure 5) shows that their binding constants, K,,are of the same order of magnitude as that of the dye, KO. The fact that KO > K, can be attributed to some specific interaction of A with the PSS benzene rings that enhances the value. These interactions play a more important role in water solution, where one observes a drastic decrease of the stacking tendency of bound dye (4, = 16-18) if compared to the free one and to that of the similar dye, AO, bound to other polyelectrolytes such as poly(acrylate)s, poly(methacrylate)s, poly(ph0sphate)s. For the last cases the A 0 q1 constant assumes values ranging from lo3 to 104.132this may be attributed to the benzene rings of PSS that favor some kind of partial intercalation of dye molecules along the PSS hai in.^,^ It is worthwhile to note that the q1 constant in methanol (ql N 8) is comparable to that in water and it is almost equal to the dimerization constant of A 0 in methan01.~ This suggests that the environment of dye bound to PSS is similar in water as in methanol and is a prevailing nonaqueous medium, similar, to some extent, to an alcoholic solution. Acknowledgment. This research has been supportyed by the Italian C.N.R.and by the Minister0 della Pubblica Istruzione. Registry No. A’, 21629-01-6; NaC1, 7647-14-5,
Investigation of the Conversion (Dealumination) of ZSM-5 into Silicalite by High-Resolution Solid-state *‘Si and *’AI MAS NMR Spectroscopy C. A. Fyfe,* G. C. Gobbi, and G. J. Kennedy Guelph- Waterloo Centre for Graduate Work in Chemistry, Department of Chemistry, University of Guelph, Guelph, Ontario, Canada N l G 2 W l (Received: June 10, 1983)
The conversion of zeolite ZSM-5, by various dealumination procedures, into Silicalite has been studied by high-field (9.4 T), high-resolution solid-state 29Siand 27AlMAS NMR spectroscopy. The 29SiMAS NMR results suggest that ZSM-5 and Silicalite are isostructural; resolved signals from up to 15 crystallographically inequivalent Si tetrahedra are observed in the 29SiMAS NMR spectra of different highly dealuminated ZSM-5 materials identical with those observed in the spectra of very highly siliceous Silicalite prepared by direct synthesis. The 27AlMAS NMR spectra indicate that the aluminum atoms are in tetrahedral coordination and the corresponding 29SiMAS NMR spectra indicate that the aluminum atoms are directly incorporated into the lattice.
Introduction
A particularly important advance in the use of zeolites as acid catalysts, especially for hydrocracking reactions in the petrochemical industry, has been the development and introduction of the zeolite catalyst ZSM-5 by Mobil Corporation.l This catalyst, which is and based On the pentasil building unit (l), shows superior performance in a wide variety of catalytic 0022-3654/84/2088-3248$01.50/0
functions? having a unique combination of catalytic activity and shape selectivity. There has been considerable discussion on the (1) (a) R. J. Argauer and G . R. Landolt, U S . Patent, 3702886 (1972). Patent. 3 941 871 (1976). (b) F. G. D w e r and E. E. Jenkins, (2) (a) S.L. Meisel, J. P. McCullough, C. H. Lechthaler, and P. B. Weisz, Chem. Tech., 6, 86 (1976). (b) C. D. Chang and A. J. Silverstri, J . Catal., 47, 249 (1977).
Published 1984 American Chemical Society
The Journal of Physical Chemistry, Vol. 88, No. 15, 1984 3249
N M R Study of ZSM-5 Dealumination
B
-100
-105
-110
-115
-120
I
-125
4'5
4'0
3'5
3'0
Ppm from TMS
25
2'0
lb
10
5
28
Figure 1. 29Si MAS N M R spectra obtained at 79.5 M H z (with respect to Me4% reference) with the corresponding XRD spectra for (A) material N 125 (892 scans, 1-Hz line broadening); (B) this material after two successive hydrothermal dealuminations of the hydrogen form at 850 "C for 24-h intervals (50 scans, 3-Hz line broadening); (C) this material after reaction with SiC14 at 550 OC for 15 h (210 scans, 3-Hz line broadening).
of Si/A1
resolution solid-state 29Siand 27AlMAS N M R at high field and showed (1) that it is possible without recourse to resolution enhancement to resolve at least nine separate 29Siresonances for a considerable number of the crystallographically inequivalent silicon atoms in the unit cell (we now consider the observation of signals from crystallographically inequivalent sites to be typical of all highly siliceous crystalline zeolites5) and (2) that a clearly resolved 27Alresonance may be easily observed whose chemical shift value (6 = -55.3) indicates that it has tetrahedral coordination, suggesting that it is present as an integral part of the lattice structure. In the present work, we report results from studies designed to further delineate the structural relationship between these two materials: We have carried out the direct conversion of ZSM-5 into Silicalite by a variety of dealumination procedures and have monitored the progress of the conversion by 29Siand 27AlMAS NMR. The results have been reported in preliminary form6s and confirmed, in part, by others.6b Experimental Section
1
N
relationship between this system and the material Sili~alite,~ which is structurally similar but is claimed to be the completely siliceous end member of the ZSM-5 substitutional series. In recent work: we have reported the investigation of a very crystalline and very highly siliceous Silicalite sample by high(3) (a) R. W. Grose and E. M. Flanigen, U. S.Patent, 4061 724 (1977). (b) E. M. Flanigen, J. M. Bennett, R. W. Grose, J. P. Cohen, R. L.Patton, R. M. Kirchner, and J. V. Smith, Nature (London), 271, 512 (1978).
29Siand 27AlMAS N M R spectra were obtained at 7 9 . 5 and 104.2 MHz, respectively, with previously described' equipment on a narrow-bore Bruker W H 400 spectrometer. Spectra are presented with appropriate line broadening as indicated without (4) C. A. Fyfe, G. C. Gobbi, J. Klinowski, J. M. Thomas, and S.Ramdas, Nature (London) 296, 530 (1982). (5) C. A. Fyfe, G. C. Gobbi, R. S.Ozubko, W. J. Murphy, and D. A. Slack, J . Am. Chem. Soc., in press. ( 6 ) (a) C. A. Fyfe, G. C. Gobbi, and G. J. Kennedy, Chem. Lett., 10,1551 (1983). (b) J. M. Thomas, J. Klinowski, and M. Anderson, Chem. Lett., 10, 1555 (1983). (7) C. A. Fyfe, G. C. Gobbi, J. S. Hartman, R. E. Lenkinski, J. H. 0'Brien, E. R. Beange, and M. A. R. Smith, J. Mag. Reson., 47, 168 (1982).
3250 The Journal of Physical Chemistry, Vol. 88, No. 15, 1984
n\
A
/
Fyfe et al.
2%
C
D
-160
-165
-1iO
-1i5
-120
,
-125
ppm f r o m TMS
45
40
3'5
30
25
20
1'5
1'0
6
28
Figure 2. 29SiMAS N M R spectra obtained at 79.5 MHz (with respect to Me& reference) with the corresponding XRD spectra for (A) material of Si/AI 18 (560 scans, 20-Hz line broadening); (B) this material after hydrothermal dealumination at 850 "C for 6 h (200 scans, 10-Hz line broadening); (C) this material after hydrothermal dealumination at 850 O C for 12 h (950 scans, 5-Hz line broadening); (D) this material after hydrothermal dealumination at 850 OC for 48 h (400 scans, 3-Hz line broadening).
resolution enhancement. Dealuminations were carried out by standard literature techniques,8-10the progress of the reactions being monitored by MAS NMR. The Si/AI ratios of the materials produced were obtained in some cases from N M R directly and also by chemical analysis. XRD spectra were recorded with a Phillips diffractometer with an automatic divergence slit. Results and Discussion Dealumination of High S i / A l Material. Figure 1 shows the 29SiMAS N M R spectra together with the corresponding XRD patterns for the starting material (Figure 1A) and the product (Figure 1B) after two successive hydrothermal dealuminations (8) (a) G. T. Kerr, J . Phys. Chem., 72, 2594 (1968). (b) ibid., 73, 2780 (1969). (9) (a) H. K. Beyer and I. Belenykaja in "Catalysis by Zeolites", B. Imelik et al., Ells., Elsevier, Amsterdam, 1980, p 203. (b) C. D. Chang, U. S.Patent, 4273753 (1981). (10) C. V. McDaniel and P. K. Maher in "Zeolite Chemistry and Catalysis", J. A. Rabo, Ed., American Chemical Society, Washington, DC, 1976;ACS Monogr. p 285.
carried out at 850 OC for 24 and 24 h, respectively, on a starting material with %/A1 = 125/1. As can be seen from the spectrum, the fine structure which we have previously reported as characteristic of the silicalite unit cell is produced while the XRD pattern remains essentially unchanged. We have recently shown that this is a completely general phenomenon-the limiting line broadening in zeolitic materials of low Si/AI ratio, which precludes the observation of site inequivalences, results from a distribution of 29Si environments from the distribution of second and further nearest-neighbor aluminum atoms in the l a t t i ~ e .Thus, ~ it is not to be expected that the multitude of resolved signals apparent in the spectrum of the product would be observed in the starting material, although some indications of fine structure may be observed: The final material still contains tetrahedral aluminum which may easily be detected by *'A1 MAS NMR. Dealumination may also be accomplished by reaction with SiCI,,: Reaction at 550 OC for 15 h yields the same overall result as indicated by the 29SiMAS N M R spectrum shown in Figure l C , although the reaction is n o t as complete. Treatment with EDTA according to standard literature procedures was found to be ineffective.
N M R Study of ZSM-5 Dealumination
The Journal of Physical Chemistry, Vol. 88, No. 15, 1984 3251
Dealumination of Low Si/AI Material. Dealumination of low Si/Al ratio material (Si/Al N 18) may be carried out hydrothermally in essentially the same manner as for the high Si/Al ratio material giving the spectra shown in Figure 2 which shows the effect of dealuminations for the various times indicated in the figure caption. This confirms the generality of the procedure and shows clearly that the spectral fine structure of the highly siliceous material may be obtained even from low Si/Al starting material. As before, there is little change in the XRD spectra over the whole range of compositions exhibited by the series. The reaction may be carried out in one step when dealumination is carried out hydrothermally for 48 h or longer at 850 "C. Comparison of Highly Siliceous Materials Produced by Dealumination and by Direct Synthesis. Figure 3 shows the 29Si MAS N M R spectra and the corresponding XRD spectra of two highly siliceous materials produced by hydrothermal dealumination of ZSM-5 samples of low and high Si/Al ratios (Figure 3, A and B, respectively) together with that of a sample prepared by direct synthesis (Figure 3C). As can be seen from the figure, all three samples show a detailed correspondence in the numbers, shifts, and intensities of the peaks indicating that they have identical structures. The corresponding XRD spectra are all identical. The very narrow lines observed (0.25-0.5 ppm) are indicative of extremely high crystallinity as well as low aluminum content so the hydrothermal dealumination is essentially nondestructive in nature under these conditions. The spectrum of the dealuminated material from the high %/A1 ratio ZSM-5 sample shows at least 15 distinct resonances which may be observed without recourse to resolution enhancement techniques. It is possible that there will be a further small but important improvement in resolution if 29Si MAS N M R spectra of this material are obtained at 120 MHz and this is being pursued to see if further details of the unit cell structure can be elucidated. 27AIMAS N M R Studies. Figure 4 shows 27Al MAS N M R spectra of samples prepared by direct synthesis which range from very low to very high Si/A1 ratios together with the corresponding 29SiMAS N M R and XRD spectra. As in the previous figures, the XRD spectra are essentially unchanged with changing aluminum content indicating that the basic cell structure is invariant within the series. However, there is a clear correlation of the splitting of the pair of peaks at approximately 45O = 28 with aluminum content as previously noted by Bibby and co-worker~.'~ These features of the XRD spectra are in general agreement with the Si/Al ratios found from chemical analysis. A well-resolved peak at ca. 54 ppm with respect to A1(H20),3+ (as) as a reference is observed in the 27Alspectrum in all cases, gradually decreasing in absolute intensity as the Si/Al ratio increases, with little accompanying change in chemical shift. The chemical shift value shows that in all cases the A1 present is in tetrahedral coordination. The lowest spectrum @/A1 N 800/ 1) shows the fine structure previously reported for material of this compo~ition.~ Inspection of the 29Sispectrum of the material of lowest Si/Al ratio (=7/1) reveals that there are clearly several peaks in the 29Sispectrum, shown resolved into Gaussian curves in the figure corresponding to the different silicon environments Si(OAl), Si(lAl), Si(2A1), and Si(3Al). The chemical shift values (-1 11"5,-104.9, -98.4, - 92.0) respectively are within the revised shift ranges previously found for a variety of zeolites." The Si/A1 ratio calculated from the 29Sispectrum by application of eq 1 is
in excellent agreement with that found by chemical analysis, indicating that essentially all of the aluminum in the sample is accounted for. Thus, the aluminum atoms must be not only in (11) (a) E. Lippmaa, M. Magi, A. Samoson, M. Tarmak, and G. Engelhardt, J . Am. Chem. SOC.,103,4992 (1981). (b) J. Klinowski, J. M. Thomas,
C.A. Fyfe, and J. S. Hartman, J . Phys. Chem., 85, 2590 (1981). (c) J. M. Thomas, C. A. Fyfe, S . Ramdas, J. Klinowski, and G . C. Gobbi, ibid., 86, 3061 (1982). (d) C. A. Fyfe, J. M. Thomas, J. Klinowski, and G. C. Gobbi, Angew. Chem., 22, 259 (1983).
B
C
-10s
-iio
-1i2
-1i4
-lie
-lis
PPm f r o m T M S
Figure 3. 29SiMAS NMR spectra obtained at 79.5 MHz (with respect to Me4Si reference) of high siliceous systems for (A) ZSM-5 of Si/Al N 20 after hydrothermal dealumination at 850 O C for 5 days (150 scans, no line broadening); (B) ZSM-5 of Si/AI 125 after hydrothermal dealumination at 850 OC for 5 days (1560 scans, no line broadening); (C) material of %/A1 800 from direct synthesis (500 scans, no line broadening).
tetrahedral coordination, as indicated by the observed chemical shift value, but must be directly>incorporated into the lattice framework, since their effect is seen in the 29Sispectrum indicating that they are directly attached (via oxygen bridges) to the silicon atoms. In fact, although the 27Alchemical shift values must be determined by the distortion of the bonds and bond angles from tetrahedral, the shift values found for a variety of structurally unrelated zeolitic materials which are known to contain a substantial proportion of five-membered rings (Table I) occur in the chemical shift range 50-56 ppm which is the low end of the observed tetrahedral "A1 chemical shift range for zeolites. This may be characteristic (although not necessarily diagnostic) of aluminum nuclei in an environment with a high proportion of five-membered rings present. This information may be of definite use in the investigation of synthetic zeolites of unknown structure where highly siliceous forms may be prepared by dealumination. As the Si/Al ratio increases, there is a decrease in intensity of the 29Sispectral peaks corresponding to higher aluminum contents in the first coordination sphere. The peaks corresponding to Si( 1Al) can be clearly observed up to quite high Si/Al ratios
3252 The Journal of Physical Chemistry, Vol. 88, No. 15, 1984
Fyfe et al.
SI(0Al)
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il B
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io
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Figure 4. 29Si MAS N M R spectra obtained at 79.5 MHz (with respect to Me@ reference), 27Alspectra obtained at 104.2 MHz (with respect to Ai(H2O),p (as) reference), and the corresponding XRD spectra for materials of different Si/Al ratios: (A) %/A1 N 7 (29Si:2010scans, 35-Hz line broadening; *'A1:1636 scans, 25-Hz line broadening); (B) %/A1 N 18 (29Si:560scans, 20-Hz line broadening; 27A1:2383scans, 15-Hz line broadening); (C) Si/A1 125 (29Si:892scans, 1-Hz line broadening; 27A1:500scans, 25-Hz line broadening); (D) Si/Al N 800 (29Si:400scans, 1-Hz line broadening; 27Al:123 000 scans, 25-Hz line broadening).
(ca. 1OO:l) as indicated in the figure (there may be a small contribution to this peak from Si(OH)(3Si) environments as these spectra, taken have similar chemical shift valuesI2). Thus, the 27N together with the corresponding 29Si spectra, indicate that the aluminum atoms are not only in tetrahedral coordination but must also be directly incorporated into the lattice.
Conclusions The results described above are in good agreement with our previous findings4 regarding the occurrence and lattice siting of aluminum atoms in Silicalite and demonstrate that it is isostructural with ZSM-5 by the conversion of the latter to the former by a variety of dealurnination procedures. ZSM-5 is defined by patent over a very large range of Si/Al ratios (greater than 10). Indeed, it may be difficult to prepare Silicalite, the theoretical end member of the series, by direct synthesis since readily available reagents contain traces of aluminum. Further, the definition will be operational in nature; whether or not 27Al is detected will depend on the technique used. We estimate that high-field 27Al
(12) (a) G. E. Maciel and D. W. Sindorf, J. Am. Chem. Soc., 102,7606 (1980). (b) C. A. Fyfe, G. C. Gobbi, and G. J. Kennedy, J . Phys. Chem.., submitted for publication. (13) D. M. Bibby, L. P. Aldridge, and N. B. Milestone, J . Cutal., 72, 373 (1981).
TABLE I: Approximate Chemical Shift Values' (ppm with respect to AI(H20)63+),of the 2'AI MAS NMR Resonances Obtained at 104.2 MHz of a Variety of Zeolite Systems Containing High Proportions of Five-Membered Rings ZSM-5 54.9 Silicalite 55.36 ZSM-11 54.6 mordenite 55.1 ferrierite 54.3 'Small dependence of chemical shift on total A1 content and residual second-order quadrupolar interactions although the latter are minimized at the high magnetic field strength used. bAverage value (fine structure observed in this spectrum).
MAS N M R is capable of detecting aluminum in these systems up to a ratio of approximately 10000/1 and (based on this criterion) are attempting to prepare material by dealumination where no aluminum is detectable by NMR. The properties of this material will be investigated in detail. We are also investigating the relationship between ZSM-11 and Silicalite I1 by similar techniques and the detailed relationship between the 29Sichemical shifts and X-ray structures for a wide range of highly siliceous material~.~J~ (14) C. A. Fyfe, W. J. Murphy, and co-workers, to be submitted for publication.
3253
J. Phys. Chem. 1984, 88, 3253-3260 Acknowledgment. The authors acknowledge the financial assistance of the Natural Sciences and Engineering Research Council of Canada in the form of an Operating Grant and Strategic Grant (Energy) (CAF) and Graduate Scholarships (G.C.G. and G.J.K). The support of an Imperial Oil University Research Grant is also acknowledged. The N M R spectra were obtained at the South Western Ontario High Field N M R Centre,
Manager, Dr. R. E. Lenkinski. The authors especially thank Dr. H. Robson, Exxon Research and Development, Baton Rouge, La., for his help in providing for study the very low Si/Al ratio zeolite sample whose spectrum is presented in Figure 3A, and Drs. M. Barlow and D. Stewart, B.P. Limited, Sunbury on Thames U.K., for their help in providing the highly siliceous sample whose spectrum is presented in Figures 3C and 4D.
Infrared and Raman Spectra and Vibrational Assignments for 1-Methyluracil and Isotopic Derivatives Thomas P. Lewis, H. Todd Miles,* and Edwin D. Becker National Institutes of Health, National Institute of Arthritis, Diabetes, and Digestive and Kidney Diseases, Bethesda, Maryland 20205 (Received: August 15, 1983: In Final Form: December 20, 1983)
Infrared and Raman spectra of 1-methyluracil and seven isotopic derivatives (2H and I8O) have been measured. Polarized Raman spectra of single crystals were used to distinguish in-plane and out-of-plane modes. All the fundamental vibrations except the methyl torsion have been assigned. Product rule agreement is’satisfactory. Relevance of these results to vibrational spectroscopy of polynucleotides in aqueous solution is discussed.
Introduction Infrared (IR) and Raman spectra have played a critical role in defining the structures of various synthetic and natural polynucleotides.’ The principal emphasis in this laboratory has been on infrared spectra in the C=C and C=O region since the carbonyl and ring vibrations have proved to be sensitive to helix formation and structurally most informative. DzO has been the principal solvent because of its transparency in this region, but with improvements in instruments and techniques, it is possible to use both HzO and D,O solvents to obtain useful structural information over a wide frequency range in both IR and Raman spectra.Ieg Interpretation of the polynucleotide spectra is, of course, facilitated by a detailed understanding of the spectra of individual nucleotide bases. Several surveys of IR and/or Raman spectra of nucleotides, nucleosides, and the constituent bases have been p ~ b l i s h e d . ~ -Papers ~ by Susi and ArdS and Tsuboi et a1.6 have presented detailed studies of uracil and 1-methyluracil, respectively. Several years ago, we undertook an extensive investigation of the IR and Raman spectra of 1-methyluracil (I) and seven deuterated or I80-substituted derivatives. Only one aspect of this work, the identification of the amide-I1 band, was published.’ The molecule 1-methyluracil is more representative of the base in the (1) See, for example, (a) G. B. Sutherland and M. Tsuboi, Proc. R. SOC. London, Sec. A, 239,446 (1957);(b) E.R. Blout and H. Lenormant, Biochim. Biophys. Acta, 17,325 (1955);(c) H. T. Miles Proc. Natl. Acad. Sei. U.S.A., 47,791(1961);(d) T. Shimanouchi, M. Tsuboi, and Y. Kyogoku, Adv. Chem. Phys., 7 , 435 (1964); (e) H. T. Miles in “Procedures in Nucleic Acid Research”, Vol. 2, G.Cantoni and D. R. Davies, Eds., Harper and Row, New York, 1971,p 205;( f ) E. W.Small and W. L. Peticolas, Biopolymers, 10, 1377 (1971);(g) L. LaFleur, J. Rice, and G. J. Thomas, Jr., ibid., 11, 2423 (1972);(h) H. T. Miles in “Biomolecular Structure, Function, and Evolution”, R. Srinivasan, Ed., Pergamon Press, Oxford, 1980,p 251,and references cited in these papers. (2) C.L.Angell, J . Chem. Soc., 504 (1961). (3)A. Y. Kyogoku, S. Higuchi, and M. Tsuboi, Spectrochim. Acta, Part A , 23A,969 (1967). (4) R. C . Lord and G. J. Thomas, Jr., Spectrochim. Acta, Part A , 23A, 2551 (1967). (5) H. Susi and J. S . Ard, Spectrochim. Acta, Part A , 27A,1549 (1971). (6) M. Tsuboi, S . Takahashi, and I. Harada in “PhysicochemicalProperties of Nucleic Acids”, Vol.2,J. Duchesne, Ed., Academic Press, New York, 1973, pp 104-145. (7) H. T. Miles, T. P. Lewis, E. D. Becker, and J. Frazier, J . Biol. Chem., 248, 1115 (1973).
.x“;.-. H
6 ; A o I CH, I
nucleotide than uracil itself, since the heavy methyl group is expected to simulate reasonably well the ribose moiety insofar as the vibrational modes of the uracil base are concerned. Subsequent to our original publication,’ Susi and Ard8 published an assignment of the in-plane vibrations of I-methyluracil, in which some limited isotopic substitution data were employed. Nishimura et al.9 reported on the in-plane modes of the uracil residue in uridine 5’-phosphate; they included data from the molecule in which both nitrogen atoms in the uracil ring are replaced by lSN. Both of these studies, as well as that of Tsuboi et relied heavily on normal-coordinate analyses of the in-plane vibrations. Very recently, Szczesniak et a1.I0 reported the infrared spectra of uracil itself isolated in rare-gas matrices, together with an analysis of normal modes predicted by quantum-mechanical calculations. In view of the continuing interest in vibrational spectra of nucleotides and polynucleotides, we are reporting here our extensive studies on 1-methyluracil. Our results include (1) data on a number of isotopic derivatives that were not previously available, (2) single-crystal Raman polarization data, (3) comparisons of spectra in the C=O and C=C region between the solid and aqueous solution, and (4) a complete assignment of both in-plane and out-of-plane vibrations. Although our assignments are generally compatible with previous publications,6x8there are some notable differences where our more extensive isotopic and polarization data permit more reliable assignments. These data should also be of considerable value in verifying and refining normal-coordinate calculations. (8) H. Susi and J. S . Ard, Spectrochim. Acta, Part A , 30A, 1843 (1974). (9)Y. Nishimura, H.Haruyama, K. Nomura, A. Y . Hirakawa, and M. Tsuboi, Bull. Chem. SOC.Jpn., 52, 1310 (1979). (10) M. Szczesniak, M. J. Nowak, H. Rostkowska, K. Szczepaniak, W. B. Person, and D. Shugar, J . Am. Chem. SOC.,105, 5969 (1983).
This article not subject to U S . Copyright. Published 1984 by the American Chemical Society
