J. Phys. Chem. 1995,99, 11813-11819
11813
Kinetics of Rapid Internal Subgroup Rotation of a Group of Phenoxy Radicals As Studied by ENDOR Spectroscopy Deanna C. Hurum and Robert W. Kreilick* Department of Chemistry, University of Rochester, Rochester, New York 14627 Received: December 5, 1994; In Final Form: May 11, 1995@
Electron nuclear double resonance spectroscopy (ENDOR) has been used to study the rate of rotation of substituents at the para position of a group of phenoxy radicals. The para substituent of the molecules contain oxime groups with aliphatic substituents on the oxime carbon atom. The oxime nitrogen atom interacts with one of the phenoxy ring protons via a through space interaction. This interaction changes the electronnuclear hyperfine coupling of this proton. Rotation of the para substituent with respect to the phenoxy ring interchanges the magnetically nonequivalent phenoxy ring protons and results in characteristic line shape changes of the ENDOR spectra from these protons. Analysis of the ENDOR line shape allows one to determine the rate of rotation. Temperature dependence studies of rates of rotation allow one to determine activation parameters for rotation. These experiments show that the rate of rotation increases as the steric bulk of para substituent increases until the substituent becomes large enough to favor a nonplanar conformation. The n-electron delocalization energy drives the molecules toward planar conformations which maximize delocalization of the spin into the oxime group while steric interactions between the aliphatic chains and the aromatic ring drive the molecule toward the conformation in which the oxime group is perpendicular to the ring. The activation energy for rotations is found to depend on the relative magnitude of these two types of interactions. The activation energy decreases with steric bulk until the aliphatic group is tert-butyl in which case the steric interaction is large compared to the n-electron energy and the molecule assumes a perpendicular conformation. The activation entropy is found to be very dependant on the bulk of the substituent. These entropy changes are explained by reorganization of solvent molecules during rotation.
Introduction
I
0'
I
R=H Magnetic Resonance spectroscopy is often the best technique CH? I /I for measurement of intramolecular motions of molecules in solution or in semi-rigid glasses'$2. The rates of motion which can be measured are determined by the frequency difference of resonance signals which are interchanged by motion in the molecule. Electron nuclear double resonance (ENDOR) spectroscopy measures the NMR spectra of paramagnetic molecules through detection of changes in EPR signal intensity. ENDOR line widths are generally sharper than EPR lines with minimal overlap of signals. Motions with rates from about 103-108 s-' can be measured by ENDOR. We have used ENDOR to measure rates of intramolecular motion for the group of 2,6-di-tert-butyl,-4-(R-O-methyl-oxime) Figure 1. The molecules studied with the various R groups. The two phenoxy radicals (R-methoxime-DTBPs) shown in Figure 1. conformers exchanged by rotation discussed here are shown on the These phenoxy radicals show well resolved ENDOR lines from right. aromatic ring protons, methoxy protons, and protons of the R group. We have found that rotation of the para substituent hyperfine coupling constants of these molecules. Deuterated around the ring as well as motions within the R substituent can derivatives were synthesized and used to simplify spectra and be monitored with ENDOR. These radicals should prove useful identify resonances when needed. for characterizing the effect of the matrix surrounding the Three different types of rotational motion have been detected radicals on various types of molecular motion. Preliminary by ENDOR. Rotation around the phenoxy ring-oxime carbon investigations of the effect of a polystyrene matrix on intemal bond will be described in this paper while rotations involving motions in these radicals have been conducted in this laboratory. the R and methoxy groups will be discussed elsewhere.' An These molecules have four major hyperfine couplings. When interaction between the nitrogen atom of the oxime group with the p orbitals of the ring and the oxime group are parallel, the one of the phenoxy ring protons leads to a nonequivalence of spin is delocalized into the oxime and the nitrogen atom in the the hypefine interaction of the two meta phenyl protons. oxime has the largest hyperfine coupling observable by EPR. Rotation of the para substituent interchanges the magnetic The meta protons on the phenoxy ring, the methoxy group environment of the meta protons and leads to ENDOR line shape within the oxime, and the protons on the beta carbon also have changes which were analyzed to give the rate constants for substantial couplings. ENDOR was used to determine proton rotation. The temperature dependance of the rate constant was Abstract published in Advance ACS Absrrucfs, July 1, 1995. used to determine activation parameters. @
0022-365419512099-11813$09.00/0
0 1995 American Chemical Society
Hurum and Kreilick
11814 J. Phys. Chem., Vol. 99, No. 31, 1995 TABLE 1: Melting Points of the 2,6-Di-tert-butyl-4-(R-O-methyloxime)phenols Synthesized and List of Deuterated Molecules
Experimental Section Synthesis of Phenols and Phenoxy Radicals. Most of the compounds included in this study were previously synthesized and studied by NMR and EPR.4v5 Other compounds were prepared by the same method with the substitution of methanol for carbon tetrachloride as the solvent. Except where noted reagents are from Aldrich and solvents from Baker. A typical synthesis follows. Preparation of 4-Keto-n-dj-methy1-2,6-di-tert-butyl phenol. Acetic acid-d4 (2 mL, 0.035 mol, Cambridge Isotopes) and 2,6di-tert-butylphenol (8.49 g, 0.041 mol) were melted together, in a beaker, with a warm water bath (-40 "C). The solution was added drop wise to a round bottom flask in an ice bath containing 5 mL (0.036 mol) of trifluoroacetic anhydride. After 15 min the ice bath was removed and the solution was stirred for 24 h. The solution was then diluted with chloroform (20 mL), neutralized with a saturated sodium bicarbonate solution, and washed with three portions of water. The solution was dried with sodium sulfate and vacuum evaporated, and the remaining oil was crystallized in methanol. The solid was then recrystallized in toluene. (Methanol was used for recrystallization in most cases.) After two recrystallizations 3.59 g of slightly pinkish white crystals was obtained (43% yield). Synthesis of keto-phenols with other R groups had yields ranging from 2 1% to 66%. Preparation of 4-(d3-methyl-O-methyloxime)-2,6-di-tert-butyl phenol. In a 50 mL round bottom flask 558 mg (2.3 "01) of the previously prepared keto-phenol and 178 mg (2.2 "01) of methoxylamine hydrochloride were mixed. They were then dissolved in 10 mL of methanol, and pyridine was added drop wise until the reactants went into solution. (About 4 mL was added.) The solution was refluxed for 20 h. After allowing the solution to cool it was poured into water, extracted into ether, and washed three times each with 2 M HCl, water, and saturated sodium bicarbonate. The solution was vacuum evaporated to an oil which crystallized slowly from aqueous methanol. White flakes (330 mg) were obtained (56% yield). Other oxime synthesis had yields ranging from 10% to 95%. Preparation of 4-(R-O-methyl-d~-oxime)-2,6di-tert-butylpheno1 was done by the substitution of methoxyl-d3-amine hydrochloride (Regis Chemical) for the undeuterated reagent, and the appropriate keto-phenol in the procedure above. The new compounds and compounds with deuterated methoxy groups which were used in this study are listed in Table 1. Compounds with protonated methoxy groups were made for all of the R groups. The radicals were prepared by oxidizing the corresponding phenol with PbO2 in a solution of toluene. The sample was then degassed with 6-8 cycles of a freeze-pump-thaw method until a pressure of about Torr could be maintained. Concentrations of the samples were estimated by comparison of EPR spectra against TEMPOL concentration standards and
by color intensity. Samples were used at concentrations between 0.5 and 0.01 mM. Equipment and Analysis. EPR spectra were run on a Bruker ER200D spectrometer interfaced to an IBM type PC for data acquisition. A Bruker B-VT-1000 temperature controller was used for variable temperature experiments. This unit maintained the temperatures within f l K of a calibrated value. ENDOR spectra were run on a modified EPR spectrometer using a Bruker ER 200 ENDOR cavity. The rf coil insert was made as previously described with 13 turns of copper wire.6 An EN1 rf amplifier driven by a Hewlett-Packard rf signal generator for FM modulation and a Phillips rf sweep generator were used for generation of the rf signal in the cavity. The microwave frequency was read with a Hewlett-Packard 5342A frequency counter and the field calibrated with a Bruker ER035M gaussmeter. Data were acquired by a program written in the ASYST programming language. The microwave power was set to about 2-4 mW while the rf power was set to 90-100 W for a typical ENDOR experiment. ENDOR experiments were conducted at a series of microwave and rf powers to monitor possible coherence effects and power broadening of the lines7 The powers used in our experiments had a negligible effect on the line shape. ENDOR spectra were taken at a series of radical concentrations to monitor the effect of spin exchange on the ENDOR line shape. The lines in the ENDOR spectrum were uniformly broadened by spin exchange as the radical concentration was increased. The concentrations selected for the experiments were large enough to yield good signal to noise without undue broadening from the spin-exchange reaction. Data analysis and spectral simulations were conducted with programs written in ASYST. The effect of the interchanges of nonequivalent nuclei on ENDOR spectra were simulated using the modified Bloch e q u a t i ~ n s . *The ~ ~ simulated ~'~ spectrum were compared with the experimental spectrum to extract the lifetimes and rate constants (k = lh).To use the modified Bloch equation approach, a suitable value for the natural line width (1/T2) must be found for use in the simulations. The most stringent method is to extrapolate a minimum line width at minimum rf intensity and minimum c~ncentration.~ A more practical and often-used method is to use a reference signal which is not undergoing exchange to approximate the line width.'' In this work a reference signal was used, as the line widths were almost independent of rf power over the available power range of our system. Molecular modeling and semiempirical calculations were carried out with HyperChem on an IBM type PC. We used the MM+ force field, a modified version of MM2,I2 for forcefield calculation of nonbonded interaction energies of the phenols.
Results and Discussion The ENDOR spectra of this group of phenoxy radicals show signals from the aromatic ring protons, the R group protons, the methoxy protons, and the tert-butyl protons. A typical ENDOR spectrum for n-nonyl DTBP is shown in Figure 3. Assignments of the individual resonances were made using a combination of selective deuteration and comparison with previous NMR ~ t u d i e s .The ~ largest ENDOR shift is from the meta ring protons in all of the radicals except radical 1, in which the C-H proton has the largest shift. The shifts and coupling constants of protons on the R group, the methoxy protons are of the same order of magnitude, and these lines were overlapped in some spectra. Compounds in which the methoxy group was deuterated were synthesized to remove any ambiguity in the
Kinetics of Rapid Internal Subgroup Rotation
J. Phys. Chem., Vol. 99, No. 31, 1995 11815
1.80"
Figure 2. Rotational geometries of the phenoxy radicals.
10.00
20.00
15.00
(MHz)
nonyl methoxime DTBP ENDOR spectrum at 190 K meta
I
10.00
15.00
.
.
.
.
.
. 20.00
(MW
nonyl d3 methoxime DTBP ENDOR spectrum at 190 K Figure 3. ENDOR spectra of deuterated and undeuterated nonylmethoxime DTBP's. The proton Zeeman frequency is denoted by YH. The central pairs of lines are from the o-tert-butyl groups and distant R group protons. The hyperfine couplings (separation between the pairs of lines) are as follows: meta, 5.14 and 4.77 MHz; R group @ carbon), 2.92 and 2.68 MHz; methoxy, 2.44 MHz; R group ( y carbon); 0.41 MHz; o-tert-butyl; 0.18 MHz.
assignment of these lines. ENDOR lines from the tert-butyl protons and more distant protons on the R group had smaller shifts which were somewhat overlapped in the center of the spectrum. ENDOR spectra were taken over the temperature range between about 175 and 230 K. The highest temperature at which an ENDOR spectrum could be observed depended on the structure of the radicals. In general, the radicals with larger R groups gave spectra at higher temperatures as the overall rotational correlation time was longer for these compounds. The ENDOR spectra from the majority of these radicals show the meta aromatic protons are nonequivalent at lower temperatures. The radicals in which R was isopropyl, sec-butyl, or tert-butyl had a single ENDOR line from the meta protons at all temperatures for which resolved ENDOR spectra could be observed. Table 2 contains the hyperfine couplings of these
compounds at the lowest temperature for which clear ENDOR could be obtained (175-185 K). One expects the two meta ring protons to be nonequivalent if the oxime group directly polarizes the closest ring proton but does not affect the more distant ring proton. The nitrogen couplings, at 220-225 K, were obtained by simulation of EPR spectra with the proton couplings found by ENDOR. The EPR spectra were broadened by both the alternating line-width effect and by a decrease in the overall rotational correlation time at low temperatures, and the spectra were poorly resolved. As a consequence, we were unable to determine if the nitrogen coupling constants were temperature dependent. The spin density at the meta position is small and negative and the coupling constants of the two meta protons are normally identi~al.~ The meta-proton couplings can be made nonequivalent if one meta proton interacts with an atom in the para substituent which is close to one meta proton but distant from the other proton. Nonequivalent hyperfine couplings from meta protons have been reported for sterically hindered galvinoxyl type radicals in which steric interactions twist the aromatic and quinone rings so that the meta protons of the quinone ring are noneq~iva1ent.I~Figure 2 shows three representations of structures of the R-methoxime-DTBP radicals. In structures I and 111, the aromatic ring and C=N group lie in a plane, while in structure I1 the C=N group is perpendicular to the plane of the aromatic ring. The nitrogen atom of the oxime group is 2.58 8, from one of the meta protons and 3.95 8, from the other meta proton in the planar structures. The oxygen atom of the oxime group is 3.81 8, from one meta hydrogen and 4.75 8, from the other in the planar structures. The meta protons are equidistant from both the nitrogen and the oxygen in the perpendicular structure. The closest nitrogen-proton separation is within the distance in which hydrogen bonding has been reported for aromatic protons and heteroatoms using crystal structure data,14 and an unpaired spin at the nitrogen should interact with the nuclear spin and affect the hyperfine coupling constant. Evidence of a strong through-space interaction has been reported in other compounds in which an unusual interaction between closely spaced nitrogen atoms has been re~0rted.I~ In the case of the phenoxy radicals reported in this paper, the nonequivalence of the two meta ring protons' hyperfine interaction depends on the geometry of the para substituent. The environment of these protons can be averaged by rotation about the bond between the para position and a carbons (angle 6). If the molecules are locked in a fixed conformation with no rotation about the para carbon and the a carbon of the oxime group, then one expects to observe two ENDOR lines from the nonequivalent ring protons. If rotation about this bond becomes rapid, the two ring protons are interchanged and the ENDOR lines will be averaged. A typical example of the effect of rotational exchange on the ENDOR lines from the meta protons is shown in Figure 4. This figure shows both experimental and simulated spectra for the hexylmethoxime DTBP. The spectra simulated with the modified Bloch equations were in good agreement with the experimentalspectra in each case. We used a two-site model for the simulation in which the molecules rotate from one planar conformation to a second planar conformation with a change in 6 of 180" (Figure 2). The conformation with 6 = 90" is apparently the transition state for this type of rotation. At low temperature, rotation is slow and one observes separate ENDOR signals from the aromatic proton on the side of the ring near the oxime group and the distant aromatic proton. As the temperature increases, the separate lines broaden, overlap, and then coalesce into a single line. This behavior is explained
Hurum and Kreilick
11816 J. Phys. Chem., Vol. 99, No. 31, I995 TABLE 2: Hyperfiie Couplings of the Phenoxy Radicals in MHz R group nitrogen meta ret?-butyl H
methyl ethyl n-propyl
n-hexyl
n -n on y 1
n-pentadecy1 isopropyl isobutyl sec-buty1 ret?-butyl
a
5.20," 4.82" 5.15," 4.15" 5.14," 4.16" 5.13," 4.11" 5.13," 4.75" 5.14," 4.11" 5.13," 4.15" 4.91 5.11,"4.74" 4.98 5.08
11.40 10.36 10.39 10.41 10.36 10.36 10.36 10.36 10.36 10.36 2.41
0.19 0.18 0.18 0.18 0.14 0.18 0.18 0.18 0.19 0.16 0.17
methoxy
R group
2.83 2.42 2.44 2.44 2.47 2.44 2.45 2.21," 1.88" 2.49 2.22," 1.98" 0.31
6.23 4.98 2.11 2.81," 2.69," 0.31 2.94," 2.68," 0.42 2.92," 2.68," 0.41 2.94," 2.68," 0.39 1.23,0.60 3.31," 2.49," 0.69 1.12,0.56,0.18 0.35
Indicates pairs of exchanging resonances. simulated
simulated 210 K 4
B
205 K
...........
simulated
experiment
methyl 7 0.0035 0.004 0.0045 0.005 0.0055 0.006 Inverse Temperature (1K) Figure 5. Arrhenius plots for rotation of para substituents. The phenoxy radical R groups are designated on the diagram. ,
-
experiment 16.0
16.5
17.0
17.5
(MZ)
Figure 4. ENDOR spectra of n-hexylmethoxime DTBP at various temperatures as shown to the left of each spectrum. The simulated spectrum is shown above each experimental spectrum. by an increase in the rate of rotation about the bond between the para ring carbon and the a carbon of the para group as the temperature is increased. The temperature dependence of the rate constants was used to determine activation parameters from Arrhenius plots. We estimate the error in activation energies and enthalpies to be f l kcdmol, while the error in activation entropies is estimated as f 2 eu. The Arrhenius plots are shown in Figure 5 while the corresponding rates, activation energies, and thermodynamic parameters are given in Table 3. The energy of various conformers which are available to these radicals depends on the electronic energy and energy from steric repulsion between nuclei in the para substituent and the aromatic ring. The electronic energy should be similar for all of the molecules with a minimum when the oxime group is coplanar with the aromatic ring to maximize n overlap. The steric energy depends on the nature of the R group and its interaction with the aromatic ring. The nitrogen coupling constant of the oxime group provides a measure of spin delocalization from the aromatic ring to the oxime group. The nitrogen coupling has a maximum value when R = H (1 1.4 MHz), and a value close to 10.4 MHz for all of the other radicals except the radical with R = tert-butyl where the nitrogen coupling falls to 2.41 MHz. These nitrogen couplings indicate that the aromatic ring and oxime group are nearly planar in all of the compounds except
.
,
.
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.
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.
,
'
TABLE 3: Thermodynamic Parameters and Rates of Rotation at 180 K AI?
A@
Ast
rate
R group
(kcavmol)
(kcaVmo1)
(cam< mol)
(Hz)
methyl ethyl Propyl n-hexyl n-nonyl n-pentadecyl isobutyl
12.6 9.4 9.4 10.1 10.0 8.1 8.6 6.4
12.2 9.0 9.0 9.1 9.6 8.3 8.2 6.0
11.6 9.3 10.9
2 4000 9500 2500 600
H
12.5 8.8 2.6 1.9 -1.9
lo00 900 3100
the radical with R = rert-butyl. The tert-butyl methoxime DTBP has a very large R group, and the steric energy necessary to form a planar conformation is apparently larger than the electronic energy gained through conjugation of the oxime group. The ground state for this molecule apparently has a conformation with angle 8 near 90" with the tert-butyl group over the top of the aromatic ring (structure I1 of Figure 2). The nitrogen and methoxy proton couplings are about a factor of 5 smaller in this radical compared to the other radicals, indicating the loss in conjugation as the para substituent twisted into a perpendicular conformation. This radical is a different color (green) than all of the others (purple), indicating a higher energy optical transition due to the loss in c~njugation.~In this perpendicular conformation the nitrogen is equidistant from each meta proton and equivalence is observed. The radical with R = H has the lowest rate of rotation due to a large activation energy and large positive entropy. Steric bulk is minimal in this compound and the energy to take the
Kinetics of Rapid Internal Subgroup Rotation molecule from one planar conformer, through the nonconjugated transition state to the other planar conformer reflects the electronic energy lost as the molecule moves to the nonconjugated transition state. The H substituent has a minimal steric interaction with the aromatic ring, and this radical does not gain much energy by twisting to remove steric interactions. The activation energies and entropies of the other molecules decrease as the steric bulk of the R groups increases. As the steric interactions between the various R groups and the aromatic ring increase, the energy from the loss of conjugation is offset by the energy gained as the molecule twists to minimize steric repulsion. The entropy change from 4-11.6 eu when R = H to -7.9 eu when R = isobutyl probably reflects the increased steric interaction of the bulky R groups with solvent molecules and with the aromatic ring. Large positive or negative entropy values have been interpreted as strong solvent effects.I6 The positive entropy would indicate better solvation in the transition state. Studies of benzene in solution suggest that the molecules prefer to stack edge to face.I7 A planar methoxime compound may be a good target for an organized solvent shell with molecules "pointing" to the ring. Toluene molecules may stack along the ring in such a manner, in a similar fashion as that found for benzene molecules. The R group chains sweeping through solvent to a perpendicular transition state would disturb this organization, giving a favorable entropy term for this motion.' Larger R groups near the ring should have less solvent organization in the original planar conformer, and the activation entropy difference between the planar starting conformer and the perpendicular transition state should decrease as the steric bulk increases. This trend is generally observed for this group of radicals. The activation entropy should also fall as it becomes more difficult to force bulky alkyl groups past the aromatic ring due to decreased rotational freedom. We have conducted preliminary experiments using n-pentane as a solvent which indicate that solvent reorganization has a strong effect on the motion of these molecules. ENDOR studies of the n-propyl radical in n-pentane gave an activation energy about '12 that of the compound in toluene and a negative entropy of -16.2 eu compared to 1-12.5 eu in toluene. The ENDOR spectra of the isopropyl and sec-butyl compounds show a single line from the aromatic protons at all temperatures where resolved spectra could be observed. Either these molecules exist as a single conformer or the rate of rotation is too rapid to be observed by ENDOR and only the averaged signal is observed. The nitrogen hyperfine coupling constants for these compounds are identical to those from other radicals with aliphatic R groups, indicating that the ground state has a planar conjugated conformation. These two compounds have the greatest steric bulk near the ring of all of the molecules, except the tert-butyl derivative, as two methyl groups are bound to the /? carbon atom. One expects the energy from steric repulsion to be very large for these compounds with a consummate lowering of the activation energy and entropy. As a consequence, rotation should be very rapid on the ENDOR time scale. To determine a more quantitative relation between the activation energies and steric bulk, we conducted molecular mechanics calculations to determine nonbonded repulsive energies. The n-electron electronic energies (En)should depend on rotational angle 8 but should be nearly identical for any member of this group of radicals at any constant value of 8. The n-electron energy should be at a minimum when the oxime group is coplanar with the phenoxy ring (6 = 0") and at a maximum when the oxime group is perpendicular to the ring
J. Phys. Chem., Vol. 99, No. 31, 1995 11817 4
Electron Energy I Non-bonded Interaction E
-iw
Rotati& Angle 8
103
Figure 6. Energies as a function of rotational angle 8.
?
X.
Figure 7. Rotation angles for R groups.
(8 = 90"). The energy of the nonbonded interaction (Enbi) between nuclei in the para substituent and nuclei in the phenoxy ring will also depend on rotation angle 8. The total energy at a given angle will be the sum of E, and &bi, and the energy difference between a conformation with 6 = 61 and 8 = 62 will be given by:
The activation energy for rotation between 6 = 0" and 8 = 90" should therefore be related to the change in n energy and nonbonded interaction energy at these two angles. The nonbonded interaction energy should have a maximum value when 6 is 0" as this is the orientation in which the para substituents are closest to the aromatic ring. The n electron energies have a minimum value when 8 = 0" and the two interactions make opposite contributions to the total energy. When this is the case, one expects that the change in energy with rotation angle will be similar to the diagram shown in Figure 6. The activation energy for rotation between 0" and 90" will depend on the relative magnitude of the nonbonded interaction and n electron energies. The difference in n-electron energies (AEn ) should be independent of the R group. As a consequence, the difference in AE(8) and A,!? for the various radicals should be dominated by differences in nonbonded interaction energies. The relative geometries and nonbonded interaction energies depend on the interaction of the R group with both the aromatic ring and with the oxime group. The MM+ energy minimization changes angles w and I$ for the various phenols (Figure 7). If XI and X2 are hydrogens and X3 is a methyl group or chain, the minimum energy torsion angle I$ places the hydrogens adjacent to the aromatic ring with the X3 group directed away from the
Hurum and Kreilick
11818 J. Phys. Chem., Vol. 99, No. 31, 1995 8,
P m
I
A
A
IsoPropyl
Figure 9. Phenoxy radical with R = ethyl (8 = OO): (a) minimum energy conformation of ethyl group; (b) ethyl group rotated 180" around the a-/?bond. Plane defined by ring
H
PH
Plane defined by ring
0
I
0
45
90
135
180
225
270
315
360
Rotation Angle (0)
H
Figure 8. Nonbonded repulsive energy as a function of 8 for the phenoxy radicals with R = H, methyl, ethyl, propyl, hexyl, nonyl, pentadecyl, isobutyl, and isopropyl.
ring. Interaction of the X3 group with the oxime group leads to small changes in angle o and to some bond lengths. Nonbonded interaction energies were calculated for each molecule as 8 was varied in 5" intervals from 0"to 360". The energy of the R group was minimized with respect to molecular geometry with 8 = O", and this molecular geometry was used in subsequent calculations of nonbonded interaction energies as a function of 8. These calculations showed a dramatic difference in repulsion energies as a function of molecular structure. Figure 8 shows a plot of nonbonded interaction energy versus angle 8 for the radicals with R = H, methyl, ethyl, propyl, hexyl, nonyl, pentadecyl, isobutyl, and isopropyl. The plot has been normalized so that the lowest energy conformation of each of these molecules is zero. The compounds with R = H, methyl, ethyl, propyl, hexyl, nonyl, and pentadecyl have a maximum nonbonded interaction energy when 8 = 0" and a minimum when 8 = 90". When 8 = O", the R group and oxime group are at a minimum distance from the ring and one expects a large nonbonded interaction. The smallest R group is H and the minimum change in nonbonded interaction energy &bi (8 = 0" -,90") is found when R = H. In this case M ( 8 ) and the Al? should have maximum values as the nonbonded interaction energy is small compared to the n-electron energy. The experimentaldata have a maximum value of Al? when R = H. The nonbonded interaction energy of the molecule with R = methyl is between the energy of the R = H molecule and the straight-chain molecules whose normalized nonbonded interaction energies are almost identical. The nonbonded interaction energy for the methyl compound is smaller than that of the straight-chain compounds because when the geometric structure is optimized, the C-H bond lengths of the methyl group are made slightly shorter than the C-H bond lengths of the methylene protons. The bond angle o is 120" for the methyl compound and 118" for the straight-chain compounds. The minimum energy structure of the radicals with straightchains has the methylene protons rotated toward the aromatic ring with the chain rotated away from the ring as shown in Figure 9a. The torsion angles (4's) between the two protons nearest the aromatic ring is 120" for the methyl compound and 116.8"for the straight-chain compounds (Figure 10). The plane
(117.5'
Straightchain
R
\.
Plane defined by ring isobutyl
I
Plane defined by ring
\ 127.0'
isopropyl
cqY00a'/
60",
'\
I
120"
120" /
\.
CY
Figure 10. Torsion angles for various R groups. of the ring bisects the torsion angle in these cases with methylene protons equidistant on either side of the ring. The conformation shown in Figure 9b has the methyl (R group) rotated toward the ring. This conformation is energeticallyunfavorable as two of the methyl protons are within 1.48 8,of the meta ring proton. The carbon atom's protons are closest to the aromatic ring and should make the largest contribution to the nonbonded interaction. In this orientation, the longer chain compounds interact with the oxime group while the methyl compound has a minimal interaction with the oxime. The difference in the C-H bond lengths, bond angle o,and torsion angle 4 cause the methylene hydrogens of the straight-chain compounds to be closer to the ring (2.22A) than those of the methyl-substituted radical (2.35 A) and lead to the higher value of the nonbonded interaction energy. The experimental values of activations energies for the methyl and straight-chain compounds are identical within experimental error and we are unable to experimentally verify these calculations. The nonbonded interaction energy of the isobutyl radical is higher than that of the straight-chain radicals and the maximum nonbonded interaction energy is shifted from 8 = 0" to about 8 = 15". In this case, the second isobutyl methyl group interacts strongly with the oxime group resulting in large changes in the torsion angles (4's). The torsion angle between one of the methylene hydrogens and the ring is 40.5"while the torsion angle for the
Kinetics of Rapid Internal Subgroup Rotation 70
,
J. Phys. Chem., Vol. 99, No. 31, 1995 11819 I
A
f
50
w
slow because of the large energy barrier associated with rotation from 0 = 90" to a planar conformation. As noted in earlier discussion of this molecule, the color of the tert-butyl radical is different from the other radicals and the nitrogen hyperfine coupling is 4-5 times smaller. These data are consistent with the nonbonded interaction conformational energy calculations. Conclusion
0 0
30
60
90 120 150 180 210 240 270 3W 330 360
Rotation Angle (e)
Figure 11. Nonbonded repulsive energy as a function of 6 for the phenoxy radicals with R = isopropyl, sec-butyl, and tert-butyl.
second methylene hydrogen is 76" (Figure 10). When 0 is 0", the first methylene hydrogen is 1.99 8, from the ring proton while the second methylene proton is 2.54 8, from the ring proton. When 0 is rotated to 15", the first methylene proton is 2.22 8, from the ring proton and the second methylene proton is 2.37 8, from the ring proton. This angle gives the maximum nonbonded interaction for this compound. The maximum nonbonded interaction energy is about 2 kcal larger than that of the straight-chain compounds, and this calculation predicts a lower activation energy. The experimental activation energy is the lowest of any the radicals measured, verifying this prediction. The isopropyl, sec-butyl, and tert-butyl radicals have two or more methyl groups attached to the fi carbon atom. A plot of nonbonded interaction energy versus 0 for these compounds is shown in Figure 11. The plot for the isopropyl radical is duplicated in Figure 9 to give a scale of reference for the two plots. The plots for the isopropyl and sec-butyl radicals show nonequal maxima in the nonbonded interaction energy at 0 = 40" and 165". The torsion angles for these compounds (Figure 10) show that the methylene hydrogen makes an angle of 36.4" with respect to the plane of the ring while the methyl group has an angle of 82.9". As angle 0 is increased, the protons of the methyl group have a minimum separation from the ring proton at 0 = 40" producing the largest peak in the plot of nonbonded interaction energy as the methyl group moves past the ring. The other smaller maxima occurs when the methylene proton sweeps past the ring. The minimum nonbonded interaction is again found when 0 is near 90", and there is a local minima at 0 = 0". The nitrogen hyperfine coupling for these radicals indicates a planar conformation (6' = O"), but the large nonbonded interaction energy for this conformation should lead to a very small activation barrier for rotation. We were unable to measure the rates of rotation for these compounds and assume that the low activation barrier leads to rotations that are too rapid to measure in the ENDOR experiments. The nonbonded interaction energies calculated for the tertbutyl compound show maxima at 0 = 25" and 155". The maximum nonbonded interaction energy for this compound is about 7 times greater than that of the isopropyl radical and 70 times greater than that of the radical with R = H. In this case the nonbonded interaction energy is apparently greater than the n-electron energy and the lowest energy conformation is found when 0 = 90". The rate of rotation for this compound is very
Kinetics studies of the rotation of the para substituents of a group of phenoxy radicals show that, counter to intuition, large bulky groups rotate more rapidly than smaller groups. The energy barrier for rotation results from the sum of a n-electron term which drives the molecules toward a planar structure and a nonbonded interaction term which drives the molecules toward a perpendicular conformation. The rate of rotation increases as the nonbonded interaction term increases until this term becomes larger than the n-electron energy and the molecules become locked in a perpendicular conformation. The meta ring protons of the molecules interact with the oxime nitrogen atom which produces a small through-space polarization and makes the protons on the two sides of the ring magnetically nonequivalent when rotation is slow. This appears to be the first observation of a through space polarization from spin on a nitrogen atom affecting the hyperfine coupling constants of the ring protons of phenoxy radicals. The experimental results indicate that there is a strong solvent effect on the overall rate of internal rotation. Large entropy differences were observed as a function of the steric bulk of the para substituents. As the para substituents rotate around the aromatic ring, the surrounding solvent molecules must reorganize. The sensitivity of the rate of rotation to the local environment of solvent molecules may make these radicals useful as probes of the local environment of various type of substrates. Acknowledgment. This work was supported in part by National Institute of Health Grant GM-22793. References and Notes (1) Sandstrom J. Dynamic NMR Spectroscopy; Academic Press: New York, 1982. (2) Kurreck, H.; Kirste, B.; Lubitz, W. Electron Nuclear Double Resonance Spectroscopy of Radicals in Solution; VCH Publishers: New York, 1988 (3) Publication in preparation. (4) Kreilick, R. J. Am. Chem. Soc. 1968, 90, 5991. ( 5 ) Yamauchi, F.; Kreilick, R. W. J. Am. Chem. Soc. 1969, 91, 3429. (6) Hurst, G.; Kraft, K.; Schultz, R.; Kreilick, R. J. Magn. Reson. 1982, 49, 159. (7) von Borczyshi, C.; Mobius, K.; Plato, M. J. Magn. Reson. 1975, 17, 202. (8) Reeves, L. W.; Shaw, K. N. Can. J. Chem. 1970,48, 364. (9) McConnell, H.J. Chem. Phys. 1958, 28, 430. (10) Zhou, D.; Kreilick, R. W. J. Phys. Chem. 1993, 97, 9304. (11) Kevan, L.; Kispert, L. D. Electron Spin Double Resonance Spectroscopy; John Wiley & Sons: New York, 1976; Chapter 3. (12) Allinger, N. J. Am. Chem. SOC. 1977, 99, 8127. (13) von Gersdorff, J.; Kirste, B.; Niethammer, D.; Harrer, W.; Kurreck, H. Magn. Reson. Chem. 1988, 26, 416. (14) Taylor, R.; Kennard, 0. J. Am. Chem. Soc. 1982, 104, 5063. (15) Kirste, B.; Alder, R.; Sessions, R.; Bock, M.; Kurreck, H.; Nelsen, S. J. Am. Chem. SOC.1985, 107, 2635. Gerson, F.; Geshcheidt, G.;Knobel, J.; Martin, W.; Neumann, L.; Vogel, E. J. Am. Chem. Soc., 1992,114, 7 107. (16) Belsky, I.; Dudiuk, H.; Shvo, Y. J. Org. Chem. 1977, 42, 2734. (17) Misawa. M.; Fukunaga, T. J. Chem. Phys. 1990, 93, 3495. JP9432307
