J . Phys. Chem. 1990, 94, 3951-3958 the i(iri,ir3*)state is the result of the strong interaction among the three doubly excited configurations, which are similar in energy (Figure 5). In fact, the stabilization caused by this interaction in butadiene is so large that the energies of the 1(irz,x3*)and I ( A ~ , T ~states *) are very close. In the propeniminium cation, because the ' ( ~ 2 , ~ 3 and * ) '(~i,ir3*)states have the same symmetry, state is destabilized and they interact. As a result, the I(ir1,ir3*) is no longer close to the '(ir2,ir3*) state in energy. For propenimine, however, on the basis of only the similar excitation energies calculated for the i(ir2ir?*)and l(irI,ir3*) states of propenimine, one cannot conclude which is the lower state. Although the final results put I(irZ,irj*) below '(irI,ir3*) by 0.23 eV for propenimine, this energy order could be easily reversed, considering the fact that only 75% of the correlation energy was calculated variationally for the I(irlrir3*) state. I n contrast to the fact that the interaction between I(irZ,ir3*) and 1(iri,ir3*)destabilizes the latter, it stabilizes the former. This stabilization is one of the reasons that the excitation energy for the I(irZ,irj*) state of propeniminium is calculated (5.96 eV, MRSD-CI) to be lower than the result for the corresponding state of butadiene (6.42 eV). The other reason can be attributed to the decreased bond alternation of the C-C single and double bonds in propeniminium, which decreases the energy gap between ir2 and ir3*. This effect is lost in propenimine, which has the largest bond alternation and much higher excitation energy for '(ir2,ir3*) (7.12 eV). The bond alternation, however, apparently has only a small effect on the '(iri,ir3*) state, which is 7.82 eV in propeniminium cation and 7.35 eV in propenimine. As shown in Table VII, the excitation energy for the Rydberg state '(n,3s) of propenimine is calculated to be 6.84 eV, very close to the 6.78 eV calculated for acrolein. The excitation energy for the '(a2,3p,) of propenimine (8.1 1 eV in Table V) is also very similar to that of acrolein (8.14 eV in Table VI). These results are not suprising, because the ionization potential of propenimine is calculated to be 9.95 eV, only 0.16 eV lower than the experimental ionization potential of acrolein (10.1 1 eV in Table VII). The relative energies of the singlet and the triplet states of propenimine show a simple pattern; that is, the triplet states are always lower than the corresponding singlet states in energy in agreement with Hund's rule. This singlet-triplet energy difference,
3951
however, greatly depends on the spatial character of these states. Because of the large exchange integral, the valence-type triplet state )(7r2,7r3*)with both unpaired electrons in the same symmetry is 2.97 eV lower than the l(irZ,ir3*) in energy, as can be seen in Table V. Conclusions
Although methaniminium does not have a low-lying 2IA, state, calculations on this cation can be used as a guide to the calculations on larger molecules. The finding that Rydberg states are much higher than valence states in energy, for example, suggests that the low-lying excited states of propeniminium are valence states as well. The l(u,ir*) state, the lowest lying singlet excited state of methaniminium, becomes the second singlet excited state of propeniminium, above the '(ir2,ir3*)state. The relative energy of this state in longer chain Schiff bases and PSBR could be higher than both the singly and doubly excited states, l ( i r Z , i r 3 * ) and I(irl,ir3*). This expectation, of course, is subject to further confirmation. For propenimine, it is found that the low-lying singlet excited states are due to the lone-pair electrons on the N atom. The singly and doubly excited states of the ir ir* excitations are close in energy, with the singly excited state being slightly lower. While excitation energies for all excited states, of a Schiff base decreases with the increasing chain length, the excitation energy of the singly excited I(ir2,~3*)state is greatly stabilized by protonation. The introduction of the positive charge to the molecule naturally stabilizes the ionic structure of this state, a result that is in agreement with the experimental results of PSBR. Population analyses on methaniminium and propeniminium show that the positive charge is largely located on the carbon atom that is next to the nitrogen. This charge localization is mostly achieved in the ir space, in which a pair of electrons can be considered as a lone pair on the nitrogen atom and the positive charge is distributed on the hydrocarbon portion of the molecules. The electron localization to the nitrogen atom in the u space is small.
-
Acknowledgment. This work was supported by grant PHS R O l G M 34081-05 from the National Institutes of Health.
Effect of Solvent on Intramolecular Reorientation in 2,2-Diphenyl+( 2,4,64rinitrophenyl) hydrazine Randy L. Tyson and John A . Weil* Department of Chemistry, University of Saskatchewan, Saskatoon, Saskatchewan, S7N 0 WO, Canada (Received: October 3, 1989)
Intramolecular reorientation in molecules of 2,2-diphenyl-1-(2,4,6-trinitrophenyl)hydrazine in dilute liquid solution has been studied by 'Hand 13CNMR spectroscopies. The temperature dependence of the rate parameter for the reorientation process was determined in 10 deuterated solvents, from line-shape analysis of the picryl proton and carbon resonances. The derived configurational mean lifetimes yielded the activation thermodynamic parameters, including the standard molar Gibbs free energy change AG* for the process. The parameter AG* correlates with a parameter (Taft's 0) measuring the relative ability of solvent molecules to form intermolecular hydrogen bonds with solute molecule protons. The nature of the reorientation process, including the structure of the transition state, is discussed.
I. Introduction Dynamic processes in liquid solution have been extensively studied by use of N M R spectroscopy over the past three decades.' *To whom correspondence should be addressed.
0022-3654/90/2094-395 1$02.50/0
However, studies of the effect of solvent on intramolecular reorientation processes in liquid solution have been relatively few in number.2 (1) For example, see: Binsch, G.; Kessler, H. Angew. Chem. 1980, 19.41 1. Mann, B. E. Prog. Nucl. Magn. Reson. Spectrosc. 1977, I I 95.
0 1990 American Chemical Society
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The Journal of Physical Chemistry, Vol. 94, No. 10, 1990
In 1964, evidence for intramolecular dynamics in 2,2-diphenyl- 1 -( 2,4,6-trinitrophenyl)hydrazine(hereafter referred to
0
\
H
N=O
Tyson and Weil TABLE I The Chemical Shift" 6 of the 1,3,5-Trifluoro-2,4-dinitrobenzeneProton Resonance at 294 K in Various Solvents with Respect to the Internal Standard TMS and the Hydrogen-Bond Acceptor Parameter B of Each Solventb solvent 1. chloroform-d
2. dichloromethane-d2 3. bromoform4 4. benzene-d6 5. nitromethane-d3 6. acetic-d4 acid 7. acetonitrile-d, 8. methanol-d., 9. acetone-d6 10. pyridine-d5 11. dimethykf6 sulfoxide
simply as DH2-H, in the notation of Chen et aL3) was observed as temperature-dependent line shapes of the trinitroaromatic (picryl) proton resonances in the 60-MHz 'H N M R spectra of DH2-H in di~hloromethane.~It was proposed that the energy barrier to reorientation is due at least in part to an intramolecular hydrogen bond between the amino proton and one of the nitro groups ortho to the amino p ~ s i t i o n .Evidence ~ for the existence of such an intramolecular hydrogen bond has now been obtained from the X-ray crystal structure of DH2-H5,6and related compounds.'-* In the lowest energy configuration in liquid solution, the hydrogen-bonded o-nitro group is thought to be coplanar with the picryl ring, with the other o-nitro group rotated with respect to the ring to reduce steric interactions with the diphenylamino group.s8 The intramolecular reorientation process is then thought to involve the breakage of the intramolecular hydrogen bond and hindered rotation about the N(,,/ring-carbon bond. The p-nitro group is considered to remain coplanar with the picryl ring throughout the process. Inspection of a molecular model of DH,-H discloses that there is ready accessibility for most solvent molecules to the amino proton. The nature of the transition state was explored by using Huckel molecular orbital calculations on simpler compounds related to DH2-H.4,9 It was postulated that the reorientation process occurs through a hindered rotation about the 1-nitrogen/ring-carbon bond. If the plane formed by the atoms H-N-C is rotated by 90" with respect to the picryl ring to reach the transition state, both o-nitro groups should lie in the picryl ring plane. To reach such a transition state, the intramolecular hydrogen bond must be broken. The possibility that the r~ bonds about the I-nitrogen, which form a planar Configuration in the lowest energy state, form a pyramidal structure in the transition state was s ~ g g e s t e d . ~ Herein, the results of a study of the intramolecular dynamics of the hydrazine in 10 solvents of varying hydrogen-bonding (proton-accepting) abilities are presented. The temperature dependence of the rate parameter for the reorientation process for DH,-H in each solvent was determined by analysis of the 3- and 5-picryl proton and carbon magnetic resonance line shapes. Attempts Mere made at correlating the resulting activation (2) For example, see: Woodbrey, J. C.; Rogers, M. T. J . Am. Chem. SOC. 1962, 84, 13. Whittaker, A. G.; Siegel, S . J . Chem. Phys. 1965, 43, 1575.
Spaargaren, K.; Korver, P. K.; van der Haak, P. J.; de Boer, Th.J. Org. M a g . Reson. 1971, 3, 615. Drakenberg, T.; Dahlqvist, K.-I.; Forsen, S. J . Phys. Chem. 1972. 76, 2178. Staps, R. J . F. M.; Scheeren, J . W.; Pijpers, F.W.; Nivard. R. J. F. J . R . Nerh. Chem. SOC.1979, 98,445. Bean, J. W.; Nelson, D. J.; Wright, G.E. Biochem. Pharmacol. 1986, 35, 101 I . (3) Chen, M M.; Sane, K. V.; Walter, R. I.; Weil, J. A. J . Phys. Chem. 1961, 65, 713 (4) Heidberg, J.; Weil, J . A. Janusonis, G. A,; Anderson. J . K. J Chem. Phys. 1964, 41. 1033. ( 5 ) Wang, H. Ph.D Thesis, University of Regina, Regina, Canada, 1989. (6) Wang, H.; Barton, R. J.; Robertson, B. E.; Weil, J . A,; Brown, K. C. J . Inclusion Phenom.. in Dress.
(7) Wang, H.; Barton,'R. J.; Robertson, B. E.; Weil, J. A,; Brown, K. C. Can. J . Chem. 1987, 65, 1322. (8) Flippen-Anderson, J. L.; Dudis, D. S. Acta Crystallogr. 1989, C45, 1101.
(9) von .Jouanne. J.; Heidberg, J . J . A m . Chem. So?. 1973. 95, 487
'6 7.165 7.214 7.314 5.183 7.459 7.469 7.457 7.698 7.877 7.997 8.147
a 0.00 0.00 0.00 0.10 0.20 0.21 0.3 1 0.41 0.48 0.64 0.76
a From 300-MHz 'H NMR spectroscopy. From ref 18, with the exception of acetic-d., acid. 'In ppm from TMS.
TABLE 11: Wavelengths,,A of the Absorption Maximum for DH,-H and the Concentration of DH,-H in Various Solvents solvent ~ n l mnm concn, M chloroform 318 0.015 f 0.001 dichloromethane 319 0.006 f 0.002 0.017 f 0.002 0.028 f 0.002 0.043 f 0.002 0.1 13 f 0.005 bromoform 328 0.018 f 0.002 benzene 315 0.023 f 0.001 nitromethane a 0.018 f 0.002 acetic acid 318 0.015 f 0.001 acetonitrile 322 0.051 f 0.001 methanol 320 0.0027 f 0.0005 acetone 325 0.034 f 0.001 pyridine 419b 0.063 f 0.014 dimethyl sulfoxide 335 0.069 f 0.008 "Not observable due to strong absorption by the solvent in this region. 6See discussion in text.
thermodynamic parameters with various solvent parameters cited in the literature.I0-'* One of these is the parameter19 @ of the solvent (see Appendix), which provides a measure of the relative abilities of solvent molecules to form hydrogen bonds with protons of a solute molecule. 11. Experimental Section
Materials. The NMR solvents used here were all perdeuterated. Pyridine-d5, methanol-d4, acetic-d4 acid, acetonitrile-d,, dichloromethane-d2, and bromoform-d were obtained from the Aldrich Chemical Co. and used without further purification. Chloroform-d, acetone&, dimethyl-d, sulfoxide, and benzene-d6, each containing 1% (v/v) tetramethylsilane (TMS), were obtained from MSD Isotopes and used without further purification. The nitromethane-d3 was thrice distilled before use. For those solvents not already containing TMS, a small amount was added to the samples for use as an internal chemical shift standard. Each sample was placed in a 5-mm-0.d. quartz NMR tube, and dissolved oxygen was removed through three freeze-pump-thaw (10) Kosower, E. M. J . Am. Chem. SOC.1958,80, 3253. (1 1) Brownstein, S. Can. J . Chem. 1960, 38, 1590. (12) Dyall, L. K. Speclrochim. Acta 1961, 17, 291. ( I 3) Allerhand, A.; Schleyer, P. v. R. J . Am. Chem. SOC.1963, 85, 371. (14) Brooker, L. G. S.; Craig, A. C.; Heseltine, D. W.; Jenkins, P. W.; Lincoln, L. L. J . Am. Chem. SOC.1965, 87, 2443. ( 1 5 ) Reichardt, C. Angew. Chem. 1965, 4, 29. (16) Mayer, U. Pure Appl. Chem. 1979, 51, 1697. ( 17) Swain, C. G.;Unger, S. H.; Rosenquist, N. R.; Swain, M. S. J . A m . Chem. SOC.1983, 105, 492. (18) Kamlet, M. J.; Abboud. J. M.; Abraham, M. H.; Taft, R. W. J . Org. Chem. 1983, 48, 2871. (19) Kamlet. M. J.; Taft, R. W. J . Am. Chem. SOC.1976, 98, 377.
Intramolecular Reorientation in DHz-H 8.2
,
TABLE III: The "Active" Temperature Range AT Used in Each Solvent for the Study of the Intramolecular Reorientation of D H r H solvent AT, K 302.0-356.0 chloroform-d" 302.0-356.0 dichloromethane-d2' 282.8-345.4 dichloromethane-dzb 302.0-372.1 bromoform-do 302.0-345.2 beI"e-d6" nitromethane-d3" 302.0-350.6 acetic-d4 acid" 302.0-334.4 acetonitrile-d3" 302.0-339.8 270.1-331.1 acetonitrile-dt methanold," 302.0-339.8 acetone-d6" 300.6-338.5 acetone-dt 216.2-325.4 302.0-356.0 dimethyl-d, sulfoxideC
7.8 -
co
7.6 -
7.4 -
7.2 -
The Journal of Physical Chemistry, Vol. 94, No. 10, 1990 3953
For 300-MHz variable-temperature 'H NMR spectroscopy. For 80-MHz variable-temperature 'H NMR spectroscopy. For 75-MHz variable-temperature I3C NMR spectroscopy.
1.
7
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 I 3
P Figure 1. A plot of the chemical shift 6 (ppm from TMS) of the 1,3,5trifluoro-2,4-dinitrobenzeneproton resonance at 294 K (from 300-MHz 'H N M R spectroscopy) against the hydrogen-bond proton-accepting parameter 0 of the solvent. The data point for acetic-d4acid was interpolated from the normal regression line for the remainder of the points.
cycles at pressures between 0.04and 0.005 mmHg. Each sample tube was sealed while evacuated. Instrumentation. The 300-MHz IH and 75-MHz I3C variable-temperature N M R spectra were obtained with a Bruker AM300 N M R spectrometer operating at frequencies of 300.135 MHz for ' H and 75.472 MHz for I3C nuclei. Magnetic field stability was monitored by means of a zH field-frequency lock unit. Temperature control was supplied by a Bruker B-VT 1000 variable-temperature control unit, able to regulate the temperature to within f0.5 OC. The 80-MHz IH variable-temperature N M R spectra were obtained with a Bruker WP80 CW N M R spectrometer operating at a fixed frequency of 80.131 MHz for 'H nuclei. Magnetic field stability was monitored by means of a 2H field-frequency lock, and temperature regulation was supplied by a B-VT 1000 variable-temperature unit. Calibrations. The variable-temperature unit of the AM300 NMR spectrometer was calibrated by use of an external standard consisting of a sample of 80% v/v ethylene glycol in dimethyl-d6 sulfoxide. For the WP80 N M R spectrometer, a similar standard composed of 4% v/v methanol in methanol-d, was used. The chemical shift differences of the methyl (or methylene) and the hydroxyl proton resonances were then compared with a standard calibration curve to determine the actual probe temperature.20 Since the hydrogen-bond acceptor parameterIg p of acetic-d4 acid was unavailable, it was estimated by interpolation from the regression line (calculated using normal regression analysis2') of a plot of the chemical shift 6 of the proton resonance of 1,3,5trifluoro-2,4-dinitrobenzene, with respect to the internal standard TMS, against p for a variety of solvents (Table I and Figure 1). The value of for this solvent was estimated to be 0.21 f 0.06. The DH2-H concentrations of the N M R samples (Table 11) were determined by use of a Cary 2315 double-beam ultraviolet-visible spectrophotometer, from a calibration curve for each solvent. Each such curve was created by using normal regression analysis2' by determining the absorptivity at the absorption maximum of the x x* electronic spectral transitionzz of DH2-H
-
(20) Bruker Spectrospin (Canada), A M - N M R User's Manual, private
(22) Rappoport, Z.; Shera
over a series of concentrations of DHz-H. The concentration of DH2-H in each NMR sample was then obtained by interpolation of its absorbance on the regression line. The statistical error in the concentration was determined by using an equation23allowing the calculation of the standard deviation of a value obtained using a calibration curve. Analysis. For those samples with relatively low freezing points (
