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Natural-abundance 13C-NMR spectra of [d(TCGCG)] (1), [d(CGCGCG)]2 (2), and [d(GGTATACC)]2 (3) were measured at 90.6 MHz to obtain 13C-1H NOEs and T1 relaxation times; relaxation data were also measured at 125.7 MHz for 1 and 2 and at 62.9 MHz for 1. Analysis of the relaxation data was performed in the context of the "model-free" approach of Lipari and Szabo [Lipari, G., & Szabo, A. (1982) J. Am. Chem. Soc. 104, 4546-4559], leading to the following conclusions: (i) Optimized values for the overall correlation times of 0.9 ns for 1 and 1.4 ns for 2 are close to those predicted by light-scattering results on similar molecules [Eimer et al. (1990) Biochemistry 29, 799-811]. (ii) For the nonterminal residues, the "order parameter", S2, is around 0.8 for the protonated base carbons and 0.6 for the sugar carbons, indicating less spatial restriction on the sugar carbons (in the model-free approach, the order parameter is 1 for a rigid body and 0 for a system with completely unrestricted internal motion). (iii) The order parameters for the terminal residues vary over a wide range with the smallest values around 0.2-0.3 for the HO-13C5' and the 13C3'-OH; rational trends are seen in the variation of S2 with chain position in the terminal residues. (iv) The analysis shows that the order parameters are accurate within 15%. (v) The "effective internal correlation time", tau e, is very short for the sugar carbons (30-300 ps) and less well-defined, but probably also short, for the bases. (vi) The analysis indicates that most of the relaxation in DNA is accounted for by S2 and the tau e is so short that a good approximation to any relaxation property, P (e.g., T1, T2, 13C-1H NOE, 1H-1H cross-relaxation rate), is P = S2Prigid, where Prigid is the value for the property in a system without internal motion (the analysis assumes the same isotropic overall motion for both the rigid and flexible bodies).

to be more stable than either 6b or 6b'. Since the conformation 6b" has a dimethylamino group in a pseudoequatorial position, the observed increase in basicity is not unreasonable.The dissociation constant of the tertiary amine 6c is quite similar to that of 6b, suggesting that the two bases may have similar conformations. The basicity of the tetramethyl derivative 8b is less than that of 8a. In 8b, the axial methyl groups at the 2,6-positions may hinder solvation of +NH(CH3)2, and hence the basicity of the amino group may be reduced. Experimental Section4-Aminooxanes la,b,d,e, 2a,b,d,e, and 3 were prepared as reported.9 Preparation of compounds lc,f-n, 2c,f-n, 4a-f, 5a-f, 6a-c, 7a-c, and 8a, will be reported elsewhere.10General Procedure for the Measurement of Dissociation Constants.1 234 The amines were purified by either repeated crystallizations or distillation under reduced pressure. Methyl Cellosolve (2-methoxyethanol) was purified initially by distillation over quick lime and subsequently by fractionation using a Dufton column [bp 124 °C (760 mm)]. Distilled water, free from carbon dioxide, was prepared, and 80% 2-methoxyethanol was used as the solvent.The amine (about 15 mg) was dissolved in 80% 2-methoxyethanol (25 mL). While the solution was stirred under nitrogen, 0.05 N hydrochloric acid was added dropwise from a buret that could be read to 0.005 mL. The pH values were measured in a pH meter, precalibrated with buffers at pH 4.0 and 9.2 with a glass electrode and a saturated calomel electrode as the reference electrode. All measurements were made at 27 ± 0.1 °C. The equivalence point was determined from a plot of pH against volume of HC1 added. An average value (pKa) of one-fourth, one-half, and three-fourths neutralizations was taken, and at least two independent titrations were carried out on each compound. The pH meter was an Elico Digital pH meter (Model LI-120) with an accuracy of ±0.01 pH units.Acknowledgment. We thank Professor D. K. P. Varadarajan, Principal, PSG College of Arts and Science, Coimbatore, India, and Mr. G. Varadaraj, Managing Trustee, PSG Institutions, Coimbatore, India, for the constant encouragement and financial support. P.K.S. thanks the CSIR, New Delhi, India, for the award of a Junior Research Fellowship. K.D.B. thanks the College of Arts and Science Office of Research for partial support in the form of salary. K.D.B. also gratefully acknowledges partial funding from National Science Foundation in the form of Department Grants to purchase the XL-100 (15) NMR spectrometer (G.P. 17641) and the TT-100 PFT accessory (CHE-76-05571).

Measurements of 13C spin-lattice relaxation times (TO and nuclear Overhauser effects at 25 MHz are reported for a number of aromatic compounds and are discussed in terms of four relaxation mechanisms: dipoledipole (DD), chemical shift anisotropy (CSA), spin rotation (SR), and scalar (SC). Examples are given of compounds with carbons dominantly relaxed by all of these mechanisms. Protonated ring carbons are largely relaxed by the DD mechanism; nonprotonated ring carbons are relaxed by both the DD and SR mechanisms, with the exception of 79Br-bonded carbons, which can relax entirely by the SC mechanism. For bromine-bonded carbons, the relaxation is nonexponential since the TVs are different for the two bromine isotopes. The CSA mechanism is negligible in these compounds but is the dominant relaxation mechanism for the central acetylenic carbons in diphenyldiacetylene, as shown by experiments at 25 and 63 MHz. The large contributions of DD and SR relaxation and nearly insignificant CSA contribution for the nonprotonated carbon of toluene were approximately determined from 25-and 63-MHz experiments. Dipole-dipole relaxation of protonated aromatic ring carbons in substituted benzenes is strongly affected by ring substitution. Large or polar substituents reduce molecular tumbling, lengthening the molecular correlation time, r" thereby shortening observed TVs. Anisotropic motion has an easily observable effect on the DD contribution to 7) and can form the basis for spectral assignments, as in 3-bromobiphenyl. With phenol and aniline, strong solvent effects owing to molecular association or protonation are found and affect not only the absolute values of 7), but also the ratios of 7>n;/7>. 3089 (1972).

between the tautomers of methyl-9-hydroxyphenalen-1 -one was found to be 154 cm"1 234, in good agreement with the results from laser-excited fluorescence and fluorescence-excitation spectroscopy. 13,15 The time evolution of the probability density of the initially localized state function demonstrates that, for the symmetric case, the proton or deuteron reaches the other well at the frequencies of their respective gerade-ungerade splittings. For the asymmetric case, three-tenths of a proton, but less than three-hundredths of a deuteron, reaches the other well with each oscillation of frequency equal to the energy difference between the two lowest eigenstates, 5.03 X 1013 and 4.52 X 1013 s, respectively.Although the introduction of the methyl substituent destroys the symmetry of the double-mininum potential, the rate is fast because the barrier, the interminimal distance, and the difference in energy between the two tautomers are small for this nearly symmetric potential energy profile. This explains the NMR spectra interpreted as showing that methylnaphthazarin does not have proton exchange, but methyl-9-hydroxyphenalen-1 -one does.Tunneling in near symmetric cases deserves special attention. In symmetric cases, one can calculate tunneling from the energy difference between the gerade-ungerade pair. This is so because the proton will spend equal time in each well, and tunnelling will depend on the frequency with which the proton oscillates between the wells. When the profile is asymmetric, the fraction of the proton that leaks to the other well decreases as the energy difference between the minima increases. It has been found26 that this fraction depends on the ratio of the energy difference between the minima and the tunneling splitting in the corresponding symmetric case. In the present study the fraction of the proton that leaks to the other well is reduced from unity in the symmetric case to one-third in this nearly symmetric case, while the frequency with which the maximum leakage occurs increases by 60%.Acknowledgment. We thank Professor L. M. Jackman of the Pennsylvania State University for stimulating our interest in 9-hydroxyphenalen-1-one, Professor J. H. Busch of Villanova University for his helpful discussions, Timothy Ay of the Villanova University Computer Center, Dr. Eugene M. Fluder from Merck Sharp and Dohme, and the staff of the Villanova University Computer Center for their help with the various programs.

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13C chemical shifts of phenyl ring carbons in substituted benzenes can be used to monitor changes in charge distribution at those carbons. Strong solute-solvent interactions such as hydrogen bonding to basic substituents result in significant changes in ring carbon chemical shifts. The changes in 13C shifts are related to the electronic perturbation of the substituent and the ring system in a near quantitative manner.Studies of these solvation effects in relatively dilute solution are facilitated by the use of Fourier transform (FT) techniques. Dilution curves indicate that for groups such as -OCH3 or -COCH3 in CF3COOH, a 10-15 mol % solute concentration effectively simulates infinite dilution insofar as electronic perturbation of the solute is concerned. By use of para 13C resonances, estimates of + values can be obtained for most substituents in most media.n contrast to the extensive investigation of solvent effects on proton chemical shifts, much less attention has been paid to the effects of solvents on 13C chemical shifts. Solvent dependence of the 13C chemical shifts of methyl iodide,2 acetonitrile,2 and chloro-form3 has been reported. Extensive solvent studies have been made on 13C resonances of carbonyl carbons,4-6 which were more easily observable by the 13C instrumental methods of the early 1960's. The carbonyl carbon of acetone, for example, shows a 13C chemical shift range of some 40 ppm over a variety of solvents.4 Surprisingly little work, however, has been reported on the effect of solvents on 13C chemical shifts of substituted aromatic systems. Some work relative to the effect of solvents on the 13C chemical shifts of phenol,7 benzonitrile,8 ,/V,iV-dialkylanilines,9 and acetophenones10 has been reported. Other than

within this range. Wilcox and Leung30 have suggested that an attenuation factor of 1/2.7 is most reasonable.Using this value, the predicted inductive slope falls very close to both the experimental and field slopes in the comparison of systems I and II. This same attenuation factor, however, predicts a slope for systems I and III which is far from the field and experimental slopes. These results suggest that the cavity model is 4897 a more useful approach in predicting the relative magnitude of substituent effects.