J. Phys. Chem. 1981, 85,2034-2036
2034
with the available experimental data. In conclusion, we can say that the PMO/1 is a useful tool for studying various properties of Ps complexes and that the Ps quenching by the protonated nitrogen-containing molecules is very probably due to the Ps complex formation reaction 1.
complexes with unprotonated and protonated pyridine, pyrimidine, pyridazine, pyrazine, quinoline, and phthalazine. Obtained charge distributions of Ps complexes were compared with those of Ps and positron complexes with other organic molecules. Obtained Ps affinities seemed to explain the difference of the previously determined Ps reaction rate constants between unprotonated and protonated molecules, and among protonated molecules in water as well.
Summary Recently developed PMO/1 method was applied to Ps
Weak Intermolecular C H y O and P-H-0
Hydrogen-Bonding Interactions in Liquids
Gary J. Gerard1 mpartment of Cbmktry, William Paterson State Colkge, Wayne, New Jersey 07470
and Joseph A. Potenza' hpaftment of Chemistry, Rutgem, The State University of New Jersey, New Brvnswick, New Jersey 08903 (Received: January 13, 198 1; I n Final Form: March 20, 198 I )
Weak intermolecular interactions between the free-radical DTBN and the deuterated dimethyl phosphites (CD,O),POH and (CH30)2PODhave been examined by use of magnetic resonance techniques. Low-field dynamic nuclear polarization and spin-lattice relaxation measurements have been used to determine absolute 'H and 31Pdipolar and scalar coupling strengths. The 31Pnuclei show predominantly scalar (-80%) coupling and dipolar rates consistent with translational modulation. Proton dipolar rates are larger than those of 31P,and substantiallylarger than those observed for similar compounds and free radicals not expected to form hydrogen bonds. They are consistent with short distances of closest approach and stereospecificcollisions in which the spin-rich nitroxide group collides preferentially with methyl and P-H protons.
Introduction Magnetic resonance techniques, including spin-pin (T,) and spin-lattice (T,) rela~ation,'-~ dynamic nuclear polarization (DNP),3-6 contact ~ h i f t , ~and * ~ lNMR ~ line b r ~ a d e n i n ghave , ~ been used to probe dynamic, thermodynamic, and structural aspects of hydrogen-bonding interactions between free radicals and hydrogen donors of varying strength. With the use of these techniques, 0H-0 and C-H-0 interactions between nitroxide radicals and and donors such as acetic acid," water,5 methanol,'~~-~ c h l o r ~ f o r m have ~ ~ ~been ~ ~ - readily ~ detected and characterized. Depending on the measurements made, characterization has included determination of the mean lifetime of the H bonds, radical-receptor distances of closest approach, intermolecular hyperfine coupling constants, and formation constants of the complexes. More recently, ESR, DNP, and Tl measurements were combined4 to yield information regarding the nature and duration of 0-H-N and the somewhat weaker C13C-H-N ~
(1) Endo, K.; Morishima, I.; Yonezawa, T. J. Chern. Phys. 1977, 67, 4760-1. (2) Endo, K.; Knuettel, B.; Morishima, I.; Inubushi, T.; Yonezawa, T. Chem. Phys. Lett. 1975,31,387-91. (3) Bates, R. D., Jr.; Wagner, B. E.; Poindexter, E. H. J. Phys. Chem. 1977,81, 276-9. (4) Gerardi, G. J.; Wagner, B. E.; Potenza, J. A. J. Chem. Phys. 1978, 69, 4645-51. (5) Meise, K.; MiiUer-Warmuth, W.; Nientiedt, H.-W. Ber. Bunsenges. Phys. Chem. 1976,80,5&1-90. (6) Maler-Warmuth, W.; h e k i n , E. Mol. Phys. 1969, 17, 105-7. (7) Morishima, I.; Endo, K.; Yonezawa, T. J. Am. Chem. SOC.1971,93, 2048-50. (8)Morishima, I.; Endo, K.; Yonezawa, T. J.Chern. Phys. 1973, 58, 3146-54. (9) Sysoeva, N. A.; Karmilov, Yu.; Buchachenko, A. L. Chem. Phys. 1975, 7, 123-9.
0022-3654/81/2085-2034$01.25/0
H-bonding interactions between imidazoline nitroxide radicals and the hydrogen donors trifluoroacetic acid and chloroform. This study revealed a decided sensitivity of low-field radical-induced relaxation rates to H-bonding interactions and prompted us to examine the possibility of detecting weak CH3-0 and P-H-0 interactions in liquids using these methods. In the present study, we report DNP and low-field T1results for solutions of the free-radical di-tert-butyl nitroxide (DTBN) and the deuterated dimethyl phosphites (CD30)2POHand (CH,O)?POD.
Theoretical Background In a radical-containing solution, the spin-lattice relaxation rate of a nucleus n is given by R , = Rd R, + Rb (1) where Rd is the radical-induced dipolar rate, R, is the corresponding scalar rate, and Rb is the relaxation rate in the absence of radical (bulk relaxation rate). Measurement of R , and Rb yields the radical-induced rate, Rd + R,. At low field, in the white spectrum region of molecular motions, scalar and dipolar coupling may be separated by using DNP to give values for Rd and R,. The theory of DNP has been reviewedlOJ1 and extended to systems containing nitroxide radicals.12 Dipolar relaxation may be modulated either by translational diffusion of the spin-containing species or by rotation of a transient radical-receptor complex. These are limiting cases (no complex formation or complex forma(10) Potenza, J. Adu. Mol. Relaxation Processes 1972, 4 , 229-354. (11) Hausser, K. H.; Stehlik, D. Adu. Magn. Reson. 1968, 3,79-139. (12) Bates, R. D., Jr.; Drozdoski, W. S. J. Chem. Phys. 1977, 67, 4038-44.
0 1981 American Chemical Society
Intermolecular CH,-O
and P-H-0
Hydrogen Bonding
The Journal of Physical Chemistry, Vol. 85, No. 14, 1981 2035
tion) used to describe complex motions which are likely, in individual systems, to contain translational and rotational components. For translational diffusion, the lowfield relaxation rate is given by13 Rdt =
4~y:y,2h~~tN,/3d,3
(2)
where Tt
+
= 12?ru1uzsd~/5(ul az)kT
(3)
For rotational modulation, the corresponding expressions are13 Rdr
= ye2yn2
'TflB
/ d,6Nt
(4)
where 7,
= Vcq/kT
(5)
In the above equations, 7 is the correlation time, dt is the radical-receptor distance of closest approach, d, is the average pair radius of the rotating adduct, NB is the number of receptor atoms bound to all radical molecules, NT is the total number of receptor atoms in solution, and 7 is the viscosity. Scalar coupling may be modulated either by diffusion if no transient radical-receptor complex is formed or by the on-off effect of sticking if such a complex is forrned.l4 The sticking model yields for the scalar relaxation rate
Rc = AnZ7JVB/2(1 + ~,27,2)NT
(6)
where A, is the intermolecular hyperfine coupling constant and 7, is the scalar correlation time. In the diffusion (Hubbard) model, the scalar interaction is assumed to be very short range with the electron (+nucleus (i) hyperfine coupling constant given by (7) A , = A(dt/cj) exp[-X(rij - dJ] where rijis the instantaneous electron-nucleus separation and X is a parameter such that Adt >> 1. For this model, the low-field relaxation rate becomes
R, = 2rA2d,3/(XZD) (8) where D is the average diffusion constant of the electronand nuclear-containing species. Experimental Section DTBN was obtained from Eastman Kodak and used as received. Monodeuterated dimethyl phosphite, (CH3O)ZPOD, was prepared by the reaction of trimethyl phosphite with an equimolar quantity of deuterium oxide and isolated by vacuum di~tillation.'~The methyl deuterated compound, (CD30)2POH,was prepared similarly by using deuterated trimethyl phosphite, (CD30)3P,and water. Deuterated trimethyl phosphite was synthesized by following a literature16procedure for the preparation of ethyl phosphite modified by the substitution of CD30D for ethanol. Both deuterated products were found to be greater than 95% pure by 'H (60 MHz) NMR. Instruments and techniques for the low-field (75 G) DNP and T1measurements have been described previously.'o The ESR frequency was 210.2 MHz, while the 'H and 31PNMR frequencies were 304.5 and 123.2 kHz, respectively. All samples (5.0 cm3) were deoxygenated by six freeze-pump-thaw cycles and sealed in glass under (13)Wagner, B. E.;Linowski, J. W.; Potenza, J. A,; Bates, R. D., Jr.; Helbert, J. N.; Poindexter, E. H. J. Am. Chem. SOC.1976,98,4405-9. (14)Hubbard, P. S.h o c . R. SOC.London, Ser. A 1966,291,537-55. (15)Kosolapoff, G.M.; Maier, L. "Organic Phosphorus Compounds"; Wiley: New York, 1973;Val. 5. (16)McCombie, H.; Saunders, B. C.; Stacey, G. J. J. Chem. Soc. 1945, 380-2.
T A B L E I: Low-Field (75 G ) D N P and T,D a t a f o r Deuterated D i m e t h y l Phosphites and D T B N system TIOH, Tl"PI s
Tau.s T;;,'S
[ D T B N 1, M R ~ HS-' , M-' R p , s-' M"
u- P
~ 1 ~M-' '
Rdp, Rep, s 77, cp
M-'
P , g ern-, %OlV,
8,
(CH,O)*POD/ DTBN
(CD,O),POH/ DTBN
6.0 12.4 0.49 0.94 0.0026 720 380 +1200 70 310 1.33 1.19 3.0
12.5 10.1
0.73 1.60 0.0011 1200 500 +lo70 110 390 1.88 1.26 3.0
O b t a i n e d f r o m liquid d e n s i t y ; for DTBN, the corresponding value is 3.5 A .
vacuum. For DNP measurements, the radical concentration was 0.021 M. Unenhanced 31P spectra for the (CH,O),POD/DTBN system required 8300 traces, using a computer of average transients to obtain a signal-bnoise ratio of 4:1, while enhanced signals required 60 traces for a comparable S/N ratio. Enhanced and unenhanced 'H spectra were recorded directly for this system. For the (CD,O)zPOH/DTBN system, unenhanced 31P and lH spectra required 24000 and 192 traces, respectively, to obtain a S N ratio of -3:l. The increased difficulty of obtaining and 'H spectra for this system was due to a lower concentration of protons and to the large P-H coupling constant (-900Hz) which split both the 'H and 31PNMR signals. Ultimate 31Penhancements were determined by the ratio method? assuming that the protons showed no scalar c0up1ing.l~ Proton and 31Pspin-lattice relaxation times (T,) in the radical-containing solutions were obtained at 75 G following the method of Bates and Drozdowski.lZ Radical concentrations were determined spectrophotometrically by comparing the absorptivity of the solutions at 440 nm to a series of standard DTBN solutions in dimethyl phosphite. For protons, bulk relaxation times ( Tlo)were measured at 75 G by observing the recovery of nuclear magnetization following inversion by rapid passage. Bulk 31Prelaxation times were measured at 32.2 MHz by using a Varian FT-80 spectrometer and an external (CD3),C0 lock. Except for the bulk phosphorus systems, all samples were degassed as above; for the bulk 31Psystems, O2was removed by bubbling Ar through each sample. Molar radical-induced relaxation rates were determined as
R = (TI-'
- Tlo-')[R*]-'
where [Re] is the molar radical concentration. We estimate the accuracy of these rates to be 120%. Viscosities were measured at 23 f 1 OC with a calibrated Ostwald viscometer.
Results and Discussion Relaxation and DNP data for the deuterated dimethyl phosphite/DTBN systems are given in Table I. Observed 31Pultimate enhancements, UmP,for both compounds are (17)Scalar coupling has been observed for protons involved in hydrogen b~nding.~.'For CHC13,which forms hydrogen bonds with DTBN, contact shift measurements have demonstrated the presence of scalar coupling, while very careful DNP measurements of the CHCl,/C@/ DTBN system3 revealed that only 8% of the radical-induced relaxation for CHC13 arose from scalar coupling. For the present systems, where hydrogen-bonding interactions are expected to be weaker than those of CHC13, the scalar component of the relaxation should be even smaller.
2036
The Journal of Physical Chemistry, Vol. 85,
No. 14,
1981
TABLE 11: Low-Field ( 7 5 G ) Radical-Induced Relaxation Rates for Selected Systems
R a”__. uln. R ~ H s, - M ~ -l
. I
receptor CF,COOH CF,COOH CHCl, CHCl, CHCl, (CD,O),POH (CH,O),POD CH,COCH, CH,COCH, (EtO),PO PhPSCl, (EtO),PO PhPSCl,
radical“ s-l M-’
cP-
ref
PhIN 2PyIN PhIN 2PyIN DTBN DTBN DTBN PhIN 4PyIN PCTM PCTM BDPA BDPA
3300 3000 2100 2100 1700 650 540 500 280 280 190 190 170
4 4 4 4 3 thiswork thiswork 4 4 22 22 22 22
2800 2400 1200 1200 1000 1200 720 290 160 450 740 310 630
” T h e abbreviations represent the following radicals:
Ph IN
2PyIN
4PyIN
PCTM
BDPA
large and positive, indicating predominantly scalar coupling (ca. 80%) at phosphorus. A large 31Pscalar coupling component has been observed previously by other workerd8 for dimethyl phosphite and interpreted as arising from weak P-H.-O hydrogen bonds. The radical-induced 31Prelaxation rates, Rp, are, within experimental error, proportional to the viscosities of the solutions and suggest no strong effect of substituting deuterium for hydrogen a t phosphorus. For both systems, dipolar 31Prelaxation rates ( R a )are small and are consistent with translational modulation of the spin-spin interaction with no complex formation. Use of eq 2 and 3 yields calculated de values of 5.9 and 5.2 A, respectively, for the CH3 and P-H systems. In the absence of complex formation, the 31Pscalar coupling may be accounted for by using the Hubbard model either by exchange p o l a r i ~ a t i o n of ’ ~electrons ~~~ in the P=O bonds or by a diffusion-controlled “weak charge-transfer interaction” of the type proposed by Endoa2’ In view of the 31Presults, we assume that the proton relaxation is also modulated primarily by translation. Compared with phosphorus, the ‘H dipolar rates are larger (18) Poindexter, E. H.; Glazer, R. L. J. Am. Chem. SOC.1970, 92, 4784-6. (19) Mtiller-Warmuth, W.; Vilhjalmsson, R. Z. 2. Phys. Chem. (Frankfurt am Main) 1974, 93, 283-98. (20) Potenza, J. A.;Poindexter, E. H.; Caplan, P. J.; Dwek, R.A. J. Am. Chem. SOC.1969, 91, 4356-60. (21) Endo, K.J . Phys. Chem. 1980,84, 3 W 4 .
Gerard1 and Potenza
than would be expected simply on the basis of gyromagnetic ratios (eq 2 and 4) and, using eq 2 and 3, lead to much smaller distances of closest approach (dtH = 3.4 and 2.9 A, respectively, for the CH3 and PH systems). These distances are consistent with stereospecific collisions in which the spin-rich nitroxide group collides preferentially with the methyl and P-H protons. While shorter approach distances might be expected for atoms located on the periphery of a molecule, such was found not to be the case for bis(dipheny1ene)phenylallyl (BDPA) and perchlorotriphenylmethyl (PCTM) with a series of tetracoordinated phosphorus compounds, including diethyl phosphite.22 For these systems, in which the radicals are not expected to form hydrogen bonds, dipolar proton relaxation rates were unusually small compared with those of phosphorus, indicating that the radicals were avoiding protonated portions of the molecules. Overall, the DNF and relaxation results presented here suggest weak CH3-.0 and P-H.-O interactions arising from the stereospecific collisions mentioned above. These data are consistent with translational modulation; however, some rotational contribution implying complex formation cannot be ruled out, particularly for the ‘H rates, and very careful multifield relaxation measurements will be required to determine its extent. For comparison with the present data, ‘H low-field radical-induced relaxation rates for a variety of systems are listed in Table 11; to place all data on a common scale, viscosity-independent rates are also given. Relaxation rates for the deuterated dimethyl phosphites clearly lie between those of the CF3COOH and CHC13 systems, which are knownl4 to form hydrogen bonds with the radicals listed, and those involving tetracoordinated phosphorus compounds with PCTM or BDPA, which do not form hydrogen bonds. This provides additional, indirect evidence for weak CH,-.O and P-H-0 interactions in the present systems. Structural, thermodynamic, and spectroscopic evidence for hydrogen bonding by methyl groups has been summarized by Green.23 The number of reported examples is small, and, in many cases, the experimental observations can be explained by assuming dipole-dipole or polarization bonding. For example, dimethyl oxylate has an abnormally high melting point, and examination of its structure24revealed four short CH,-.O contacts which were interpreted as arising from association of a polar carbonyl group and a polarizable methyl group. A similar interpretation can be applied to the deuterated dimethyl phosphites and DTBN. As the molecules approach each other, there is a preferential tendency for the polar nitroxide group of the radical to collide with polar (P=O) or polarizable (CH,, P-H) groups on the receptor molecules. Since the nitroxide group contains virtually all of the unpaired electron, these interactions result in substantial contributions to the proton relaxation rates.
Acknowledgment. The authors gratefully acknowledge support of this work by the Busch Fund and thank Dr. D. Z. Denney for obtaining the bulk 31Prelaxation times. ~~
-
(22) Goetz, A. G.; Denney, D. Z.; Potenza, J. A. J. Phys. Chem. 1979, 83, 3029-32. (23) Green, R.D. “Hydrogen Bonding by C-H Groups”;Wiley: New York, 1974; p 53-7. (24) Dougill, M.W.; Jeffrey, G. A. Acta Crystallogr. 1953, 6, 831-7.
