J . Phys. Chem. 1985, 89, 4424-4426
4424
Conformational Analysis of the 2,2-Dimethylbutyl Radical by EPR Spectroscopy’ K. U. Ingold, Division of Chemistry, National Research Council of Canada, Ottawa, Ontario. Canada K1 A OR6
D. C. Nonhebel, Department of Pure and Applied Chemistry, University of Strathclyde, Glasgow, CI 1XL, U.K.
and J. C. Walton* Department of Chemistry, The University, St. Andrews, Fife, KY16 9ST, U.K. (Received: July 22, 1985)
The electron paramagnetic resonance (EPR) spectra of CH3CH2C(CH3)2CH2.and CH3CH2C(CD3)2CH2. indicate that at 95 K these radicals exist in a rigid conformation with respect to rotation about the three CB-C, bonds and that the &methyl group (of the ethyl moiety) is gauche to one methyl and to the CH2. group. Analysis of the y-H hyperfine splittings indicates bond the SOMO eclipses one of the y-methyl groups, not the y-ethyl that in the preferred conformation about the C,-C, group. The internal rotation barriers of the CD3and ethyl groups were estimated to be ca. 4 and 6 kcal/mol from the exchange broadening in the EPR spectra at higher temperatures.
A common prerequisite for there to be a sizable long-range hyperfine splitting (hfs) in an EPR spectrum is that the radical must have a frozen conformation on the EPR time scale.2 Long-range hfs’s are therefore a common feature in the spectra of radicals of fixed geometry, such as many bi- and tricyclic radicalsS2Small y - H hfs’s3 have been observed in acyclic alkyl radicals,4d but the difficulty of resolving them has limited their study and their use in determining the preferred conformations of such radicals. However, we showed recently7 that well-resolved y-H hfs’s could be observed in isobutyl and neopentyl radicals at low temperatures ( T < 130 K ) because these radicals became more or less locked into fixed conformations at such temperatures. We have now studied the EPR spectra of the larger 2,2-dimethylbutyl radical, CH3CH,C(CH3),CH2. (l),and its deuterated analogue, CH3CH2C(CD3),CH2.(2), over a range of temperatures. Our results permit a detailed conformational analysis of this radical.
Experimental Section Radicals were generated from the parent bromide in degassed solutions by bromine abstraction using photochemically produced triethylsilyl radicals’ or, at T > ca. 200 K, trimethyltin radicals. Propane was used as the solvent at 86 6 T Q 145 K and cyclopropane or tert-butylbenzene at higher temperatures. EPR spectra were recorded at 9.4G H z on a Bruker ER2OOD spectrometer. Spectra were simulated by using a modified version of Heinzer’s program.* Results and Discussion The EPR spectrum of 1 at 250 K is shown in Figure 1A. The EPR parameters are g = 2.0027, laH(2Hff)l= 22.1 G, and laH(8H,)I = 0.9 G. Thus, all eight y H are resolved but the 6-H hfs’s are less than the line width. On lowering the temperature a complex sequence of exchange-broadened spectra are obtained until T < 120 K . At these very low temperatures well-resolved spectra are again obtained (see Figure 1B,C) which indicate that (1) Issued as N.R.C.C. No. 24875. (2) King, F. W. Chem. Rev. 1976, 76, 157-186. (3) We identify the hydrogen atoms as follows: CHI-CH,-CHB-CH,.. (4) Fessenden, R. W.; Schuler, R. H. J. Chem. Phys. 1963,39,2147-2195. (5) Hudson, A,; Jackson, R. A. J. Chem. SOC.,Chem. Commun. 1969, 1323-1324. ( 6 ) Edge, D. J.; Kochi, J. K. J . Am. Chem. SOC.1972, 94, 7695-7702. (7) Ingold, K. U.; Walton, J. C. J . Am. Chem. SOC.1982, 104, 616-617. (8) Heinzer, J. QCPE 1972, No. 209.
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‘
IOG
Y
Figure 1. Low-field ( M , = -1) line (second derivative) of the 9.4-GHz EPR spectrum of the CH3CH2C(CH3)2CH2. radical (A) at 250 K in tert-butylbenzene, (B) at 95 K in n-propane, and (C) at 85 K in npropane: left, experimental spectra; right, computer simulations.
the radical has adopted a fixed conformation. The hfs’s are strongly temperature-dependent. The following y - H hfs’s (in gauss) were deduced by comparison of simulated and experimental spectra: T/K
aH(lH,,.)
aH(1H7)
aH(lHy)
aH(5Hy)
85 95
(+)6.0 (+)5.8
(+)2.1
(+)1.4
(+)2.2
(+)1.7
(-)0.7 (-10.7
At T < 120 K rotations of the two methyl groups and of the ethyl group are at the slow exchange limit.7 Radical 1 has 0 1985 American Chemical Society
The Journal of Physical Chemistry, Vol. 89, No. 21, 1985 4425
Letters
6 therefore adopted a conformation which is fixed except for the CHI. rotor. Because of its much lower barrier: the CH2. group still undergoes a rapid, though hindered, rotation. Since three large y-H hfs's are observed, 1 must adopt conformation 3 rather than 4. That is, in 3 there are three y-H's which are trans to
;=.'
I
H7 t
C"3
3
4
the CH2. group, i.d., 0 = 180" [the Ttof the ethyl group (shown) and one H,, from each methyl group (not shown)], and there are five y-H's which are gauche to the CHI. group, Le., 0 = 60" (the HTBshown and two more on each methyl group). The alternative conformation 4 has two Hyt(on the methyl groups) and six H,!, and so it does not fit the observed spectra. We believe that 3 is favored over 4 for steric reasons. That is, the gauche interaction of the 6-CH3 group (on the ethyl) with one y-CHSand the CH2. group in 3 is energetically favored over the gauche interaction of the 6-CH3 group with two y-CH3 groups in 4. In agreement with this analysis, we have shown elsewherelo that the CH2. group in c-C6HI1CH2.has a smaller axial/equatorial conformational Le., CHI. energy difference than the CH3 group in c-C6HIICHJ; is "smaller" than CH3. The three 4,are expected to have large positive hfs's, and the five H,, are expected to have small negative hfs's, as has been indicated in the above tabulation. Since the three It/t have unequal hfs's, the rotation potential of the CH2-group has three unequal "wells", with the one which yields the 6.0-G hfs considerably "deeper" than the other two. Models suggest that, of the three possible conformers for rotation about the C&,- bond, 5-7, there is least steric repulsion between CH,. and the 6-CH3group in 5. This, conformation should therefore correspond to the lowest minimum in the CHI. rotational potential. In 5 one Y,, has r#~ = 0" and this will ive rise to the 6.0-G hfs. The other two H,, have 4 = 60" (H;,) and are responsible for the 2.1- and 1.4-G hfs's. As the temperature is raised, the CHI. rotation frequency increases and the populations of confomers 5-7 become more equal. As a consequence, the 6.0-G hfs should decrease while the other two hfs's increase, just as is observed experimentally (vide supra). The EPR spectra of 1 are too weak and complex to analyze the exchange broadening that occurs at 120 3 T Z 220 K, and we therefore turned over attention to the deuterated radical, 2.
(e,),
f
(9) Kocbi, J. K. Adu. Free-Radical Cfiem. 1975, 5, 189-317. (10) Ingold, K. U.; Walton, J. C., submitted for publication in J. Am. Cfiem.SOC.
At T 3 240 K 2 has laH(2Ha)1= 22.3 G and such broade lines (AHpp= 1.3 G) that no other hfs's are resolved. At 85 K 2 has the following additional hfs: laH(lH,)I = 2.2, laH(lH,)I = 0.7, laD(lD,)I = 0.9, and laD(lD,)I = 0.3 G . The hfs's by the remaining four y-D are within the line width, satisfactory simulations being obtained with laD(4D,)I = 0.1 G. These deuterium hfs's agree, to within experimental error, with the values calculated from the H hfs of 1, viz., 0.92, 0.21, and 0.11 G at 85 K. The fact that one large H, hfs and two large D, hfs's were observed confirms our conclusion that the preferred conformation about the C,&, bond is 3. Similarly, the fact that the H, hfs of 6.0 G in 1 corresponds to a D, hfs (of 0.9 G) in 2 and that the two y-H of the ethyl group in 2 give hfs of 2.2 and 0.7 G confirms our view that 6 cannot be the lowest energy conformer (amongst 5-7) for rotation about the C&,. bond. The SOMO must eclipse one of the CD3 groups of 2, with conformer 5 being more probable than 7, in order for there to be one deuterium with 0 = 180" and 4 = 0" (Le., D;,; see 8) which gives rise to the large (0.9 G) D DA
' , D;q%&
W t
hfs. On raising the temperature two virtually separate exchangebroadening processes were observed in the spectra of 2. In the range 140-180 K the seven-line y-structure of the basic a-H tri let broadened out, leaving only the 2.2-G H, doublet due to H,,6! of the ethyl group still resolved." In the range 190-260 K a further broadening sequence transformed this doublet into the final singlet.I2 We attribute the first of these changes to rotation of the two CD3 groups and the second to ethyl group rotation. This radical therefore provides an interesting example of a very rare phenomenon: two different internal rotations which are temperature-resolved in the EPR spectra. Preliminary simulation^'^
(11) Assuming that the 0.9- and 0.3-G D hfs's are positive and that laD(4D,)I = 0.1 G is negative, as would be expected by analogy with the neopentyl radical,' then fast rotation of the CD3 groups would yield an averaged D hfs, aD(6D,) = (0.9 0.3 - 0.4)/6 = 0.13 G, which would be within the line width at 180 K. This line broadening also obscures the (-)0.7-G hfs due to the second H, (HZ) of the ethyl group. (12) In the fast exchange limit (T2 250 K) hfs's by y-D, aD(6D,) = 0.13 G, and by Y-H, aH(2H,) = (2.2 - 0.7)/2 = 0.75 G, are unresolved and lead to the broad lines referred to in the text. (13) The necessary three-jump model involves the interchange of three radicals each with 972 lines. A 400-point calculation on a VAX-11/780 computer using the modified Block equations took ca. 3 h!
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indicate barriers of ca. 4 and 6 kcal/mol for CD3 and CH2CH3 rotation, respectively. The CD3 rotation barrier appears to be normal for a methyl group rotating against what is essentially a tertiary alkyl group.l4 Very few rotation barrier for ethyl groups (14) The rotation barrier for (CH,),C+CH, is 4.7-5.2 kcal/mol; see: Low, J. P. Prog. Phys. Org. Chem. 1968,6,1-80. Pitzer, K. S. J . Chem. Phys. 1937, 5 , 413-419. Rush, J. J. J . Chem. Phys. 1967, 46, 2285-2291. For (CH3),CH+CH1 it is 3.9 kcal/mol; see: Lide, D. R.; Mann, D. E. J . Chem. Phys. 1958, 29, 914-920. Aston, J. G.; Kennedy, R. M.; Schumann, S.C. J . Am. Chem. SOC.1940, 62, 2059-2063. See also ref 15.
are known, but the EPR result with 2 is close to the ethyl barrier in CH3CH2-CH(CH3)CH2CH3which is 5.0 k ~ a l / m o l . * ~ Acknowledgment. K.U.I. and J.C.W. thank NATO for the award of a research grant without which the present work would not have been undertaken.
(15) Chen, J. A.; Petrauskas, A. A. J . Chem. Phys. 1959, 30, 304-307.
FEATURE ARTICLE Ab Initio Computational Chemistry Enrico Clementi IBM Corporation, Data Systems Division, Department 48B M S 428, Kingston, New York 12401 (Received: April 16, 1985; In Final Form: August 5, 1985)
We describe briefly a global approach to simulations of complex chemical systems, and we illustrate it with a few examples. We discuss (1) liquid water simulations with up to four-body and with vibrational corrections; (2) hydration networks in a crystal; (3) water and ion structures for DNA; (4) determination of three-dimensional structure of proteins; and ( 5 ) transport of ions through membranes. A parallel supercomputer-we have assembled-appears exceptionally well adapted for these computations and is therefore described summarily. We conclude that a facility like the one provided by our parallel supercomputersas well as the global approach-quantum chemistry, statistical mechanics (Monte Carlo and molecular dynamics), and fluid dynamics-is often necessary to simulate complex systems realistically.
I. Introduction More and more we realize that important aspects of chemical research can be simulated on digital computers, and we are becoming increasingly aware that computer simulations can be derived directly from theory, without the need of empirical parameterizations. In this work we shall address the task to realistically simulate complex chemical systems from the points of view of present theories, application softwares, and also system hardwares. To start with, we define “chemical complexity” and we summarize our computational approach. Then we shall show with examples that this approach is capable of simulating nontrivial systems. Finally we shall conc1ud.e by describing, briefly, a new supercomputer, named ICAF’, loosely coupled array of processors, which appears to be notably well adapted to the needs of computational chemistry. The complexity of a chemical system is proportional to the number of mutually dependent variables either characterizing or related to that aspect of the system we wish to model. When one deals with biological systems, this definition might not be sufficient and we might include conditions whereby structural forms can evolve from previous forms at minimal energy cost. In these evolving systems, boundary conditions and fluxes need to be added to the above definition of complexity. Equilibrium considerations are often insufficient. Depending on the complexity of the system, one uses as models either quantum mechanics or statistical mechanics; for continuous systems we turn to fluid mechanics (with thermodynamics). In biological systems an amino acid, a protein in solution, and a cell are examples of systems appropriate to the above three techniques. One can safely predict that important advances in the understanding of complex chemical systems will be obtained by at0022-3654/85/2089-4426$01.50/0
tempting to connect and overlap quantum mechanics with statistical mechanics and with fluid dynamics. Here we do not speak of a “formal” overlap, that essentially is available, but mainly of an “operational” connection, obtained, in our opinion, primarily by means of computational methods and “free” access to “supercomputers”, namely computers with the highest performance. Using contemporary computer sciences language, we propose to model complex chemica! systems with an “expert system”. Let us comment on this somewhat unconventional “expert system”. Rather clearly we must reject the use of one model only (say quantum chemistry) as “the tool” to describe complex chemical systems. Indeed our expert system is based on the assumption of a general method linking different submodels, which traditionally have been considered as somewhat “independent”. These submodels constitute and define an ordered set, and the elements of the set are linked in such a way that the output of submodel (i-1) constitutes the input of-submodel (i). This is a basic rule in our expert system; the laws of quantum, statistical, and classical mechanics are other “rules” for the expert system. Briefly, we deal with an a b initio expert system. In general, a simulation of a chemical system can be staged into few successive steps.’ At first, we start by computing the energetic and the structural characteristic of its separated molecules using as input the number of electrons and the number and type of the nuclei only; ab initio quantum chemistry is used for (1) See, for example, the monographic works by: (a) Clementi, E. “Determination of Liquid Water Structure, Coordination Numbers for ions and Solvation for Biological Molecules”; (b) Clementi, E. “Computational Aspects for Large Chemical Systems”; Springer-Verlag: New York, 1980; Lecture Notes in Chemistry, Vol. 19. (c) Clementi, E. IEM J.Res. Deu. 1981, 25, 4, 315.
0 1985 American Chemical Society
