J . Phys. Chem. 1990, 94, 1275-1279 interaction which is set up when a chloro (bromo, iodo, etc.) and a half-filled orbital are in the same plane on adjacent atoms, resulting in a diminished shuttling barrier and the consequent relaxation in the direction of the C , configuration, with increase in the rotational barrier. Thus, all aspects that have been associated with the bridging phenomenon are readily understandable with this construct.
1275
Acknowledgment. Partial financial support given to this work by the Land Nordrhein-Westfalen is gratefully acknowledged. The services and computer time made available by the computer center of the University of Bonn have been essential to the present study. Registry No. CICH2CH2’,165 19-99-6.
Electron Paramagnetic Resonance Spectra and Structures of Cu[ C2H4], Cu[ C2H4I2,and Cu[C2H4],, in Hydrocarbon Matrices‘ J. A. Howard,* H. A. Joly? and B. Mile* Division of Chemistry, National Research Council of Canada, Ottawa, Ontario, Canada K I A OR9 (Received: November 22, 1988; In Final Form: August 4, 1989)
Two mononuclear a-complexes, Cu[C2H4]and Cu[C2H4I2,have been positively identified by EPR spectroscopy from reaction of Cu atoms and ethylene at 77 K in inert hydrocarbon matrices on a rotating cryostat. The spectra of these copper(0) complexes are consistent with dative bonding for both species and with a C, structure for Cu[C2H4]and a D2 structure for C U [ C ~ H ~ ] ~ . Spectra of CU[’~CH,CH,]and Cu[I3CH2CH2I2are consistent with these assignments. A third complex is formed in both adamantane and cyclohexane that could be Cu[C2H4I2with a structure other than D2 but is more likely to be the mononuclear trisligand complex Cu[C2H413with a D3,, structure. In the absence of a well-resolved isotropic spectrum of Cu[I3CH2CH2],, this assignment must however be taken as tentative.
Introduction There have been several spectroscopic (EPR, IR, resonance Raman, and UV-visible) studies of the reaction of Cu atoms with ethylene under matrix-isolation conditions at cryogenic temper~.~ that three atures (S77 K).3-7 Ozin and c o - w o r k e r ~suggested mononuclear zero-valent copper complexes are formed with one, two, and three ethylene ligands, Le., Cu[C2H?],Cu[C2H412,and Cu[C2H4I3. Each complex displays intense visible charge-transfer and ultraviolet absorptions that red and blue shift, respectively, with increasing ethylene stoichiometry. IR bands have been assigned to the C=C stretching and CH2 deformation and wagging modes of Cu[C2H4] and Cu[C2H412 while positive stoichiometric assignment of Cu[C2H4I3 has proved difficult because of complications due to isotopic band overlap. EPR studies of the Cu/C2H4 system in argon6 have identified two spectra, one with a large Cu hyperfine interaction (hfi) assigned to Cu[C2H4]and one with a much smaller Cu hfi assigned to Cu[C2H4I2. There was no evidence of an EPR spectrum in argon that could be assigned to Cu[C2H4I3. A recent resonance Raman and infrared study in pure ethylene and ethylene/argon mixtures’ confirmed the formation of the three copper/ethylene complexes and provided sound evidence for the formation of the dimer, Cu2[C2H4I2. The stability of these complexes was found to increase in the order Cu[C2H4] < Cu[C2H412 < CU[C2H413. We have performed an EPR study of the reaction of Cu atoms with C2H4 in inert hydrocarbon matrices on a rotating cryostat ( I ) Issued as NRCC No. 30800. (2) NSERC postdoctoral fellow, 1987-1988. (3) Huber, H.; McIntosh, D.; Ozin, G. A. J . Orgummet. Chem. 1976,112, C5W54. (4) Ozin, G. A.; Huber, H.; McIntosh, D. Inorg. Chem. 1977, 16, 3070-3078. ( 5 ) McIntosh, D. F.; Ozin, G. A.; Messmer, R. P. Inorg. Chem. 1980, 19, 3321-3327. (6) Kasai, P. H.;McLeod, D., Jr.; Watanabe, T. J . Am. Chem. Soc. 1980, 102. 179-190. (7) Merle-MEjean, T.; Bouchareb, S.;Tranquille, M. J . Phys. Chem. 1989, 93, 1197-1203.
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at 77 K and report here tentative spectral evidence for the formation of Cu[C2H4I3as well as positive evidence for the monoand diligand complexes.
Experimental Section The rotating cryostat and instruments used to record and calibrate EPR spectra have been described previously.* Copper oxide enriched to 99.89% in the isotope 63Cuwas obtained from Oak Ridge National Laboratory, TN. It was reduced to the metal by hydrogen at 500 OC. Ethylene (C2H4) was obtained from Matheson, and 13CH2CH2(99 atom % 13C) was obtained from MSD, Montreal. The inert hydrocarbon matrices used were adamantane and cyclohexane and were obtained from Aldrich. Cu:C2H4:matrix ratios were typically 1:-40:500; Le., there was an excess of reactant over metal atom. While the metal and matrix fluxes were generally kept constant, the C2H4 inlet pressure was varied by a factor of IO but the exact amount of C2H4 deposited was not determined. Values of ail and g,, were obtained from directly measured transitions and are accurate to f 2 MHz and *0.0006, respectively, while a , and g , values were obtained by computer simulation and are probably accurate to about 5 MHz and 0.0015, respectively. Results Adamantane. 63Cu atoms (25 mg) and C2H4 (0.07 Torr) in adamantane (1 g) gave a dark red deposit, the EPR spectrum of which at -4 K consisted of an almost isotropic quartet 1 with a63 = 3730 MHz and g = 1.9930 and a more intense central feature 3 (Figure la) that was indicative of a species with all >> a,. This central feature was not the same as the central feature given by Cu/C2H4 in Ar.6 A satisfactory simulation of this spectrum (Figure lb) was obtained by using the magnetic parameters a , = 60 MHz, all = 152.2 MHz, g , = 2.0121, gll = 2.0021, and P, = 10 MHz., i.e., the carrier of the spectrum had (8) Buck, A. J.; Mile, B.; Howard, J. A. J . Am. Chem. SOC.1983, 105, 3381-3387.
Published 1990 by the American Chemical Society
1276 The Journal of Physical Chemistry, Vol. 94, No. 4, 1990
,
9149 MHz
Howard et ai. 50 G
-f
a
a 3130 G
3400 G
b
I' Figure 1. (a) Central feature of the EPR spectrum of Cu/C2H4deposited in adamantane at 77 K. (b) Simulated spectrum of Cu[C2H4I3based on the parameters given in the text.
Figure 3. (a) Central feature of the EPR spectrum of Cu/C2H4deposited in cyclohexane at 77 K. (b) Simulated spectrum of a 2:l mixture of Cu[C2H4],and Cu[C2H4I2based on the parameters given in the text. n
n
' 3230G
Figure 4. Central feature of the EPR spectrum of Cu/13CH2CH2in cyclohexane at 77 K at high gain, highlighting the I3C hfi about g3.
Figure 2. (a) Central feature of the EPR spectrum of Cu/13CH2CH2 deposited in adamantane at 77 K. (b) Simulated spectrum of Cu["CH2CH2I3based on the parameters given in the text. (c) Simulated spectrum of C U [ ' ~ C H ~ Cbased H ~ ] on ~ the parameters given in the text.
axially symmetric g and A tensors. The forbidden transitions labeled with asterisks in Figure l a only appeared in simulations that included a quadrupole term, P,, in the spin Hamiltonian. Since Euler angles were not needed for a satisfactory simulation of the spectrum, the principle axes of the g, A, and P tensors were close to collinear. Furthermore, only one quadrupole interaction was needed, indicating a tensor that was axial and of the form [ I O MHz, -5 MHz, -5 MHz]. As a sample of Cu and C2H4 in adamantane was gradually warmed, 1 disappeared at about 173 K and 3 became increasingly less well resolved and finally disappeared at 120 K. Reaction of 63Cu atoms with I3C-enriched C2H4 having only one of the carbon atoms enriched, Le., I3CH2CH2,in adamantane at 77 K gave a spectrum the central feature of which is shown in Figure 2a. Comparing this spectrum with the one from Cu
and CzH4revealed that the transitions associated with gll from Cu/C2H4 and Cu/13CH2CH2had the same width indicating the absence of a measurable I3C hyperfine interaction in that direction while the lines associated with g, showed evidence of a I3C hfi of about 20 MHz. The resolution was, however, not good enough to enable the exact number of ligands to be estimated. A weak spectrum of 1 from Cu and 13CH2CH2was detected, and the line shape suggested interaction of the unpaired electron with one 13C nucleus and a hyperfine interaction of 16.8 MHz. Cyclohexane. The complex powder spectrum given by 63Cu atoms and C2H4 in cyclohexane at 77 K, shown in Figure 3a, was quite different from the spectrum given by these reactants in adamantane, but was similar to although not identical with the central feature given by Cu/C2H4 in argon.6 The high-field pattern of lines was identical with Kasai's spectrum while the low-field part of the spectrum suggested two overlapping powder spectra. A satisfactory simulation of the spectrum in cyclohexane was only obtained by using the magnetic parameters of the central feature in adamantane and the following orthorhombic parameters: a l = 94.5 MHz, a2 = 132 MHz, a3 = 142 MHz, g1 = 2.0100, g2 = 2.0040, g3 = 1.989, and P, = IO MHz. Reaction of Cu atoms with 13CH2CH2in cyclohexane at 77 K gave a spectrum similar to Figure 3a but with extra lines due to interaction of the unpaired electron with 13C nuclei. This spectrum is shown at high gain in Figure 4. Comparison of the spectra from Cu/C2H4and Cu/I3CH2CH2in cyclohexane revealed that I3C coupling was only observed in the g3 direction. The pattern of lines associated with the highest field transition was readily recognized as a triplet with a3(2 C) = 41.2 MHz.
The Journal of Physical Chemistry, Vol. 94, No. 4, 1990 1277
Cu[C2H4], Cu[C2H4I2,and Cu[C2H4I3in Matrices
TABLE I: Magnetic Parameters of Copper/Ethylene Complexes in Rare-Gas (RG) and Hydrocarbon Matrices (RH)' complex
matrix RG RH RG RH
RHC
Q1
3974
84 94.5 -0 60 20
a2
03
ais0
g1
g2
g3
3974
4045
1.976
1.976
2.01 8
1.990b 1.993
132 132
156 142 43 152 0
3997.7b 3730 17 124 123
2.010 2.010
2.005 2.004
1.989 1.989
2.00 13 2.001
2.012
2.01 2
2.002
2.0047
-0 60 20
91 13
gi,
'Hyperfine interactions in megahertz. bAverage value. cCu(CO)3in CloH16has ul = u2 = 0, u3 = 225 MHz, g, = g2 = 2.0029, and (ref 11).
Discussion Spectral Assignment. Species I. The EPR transitions of this species are isotropic, and only the s-orbital contributions to the SOMO can be determined. Dividing the Cu hfi of 3730 MHz by the one-electron parameter, A = 5995 M H z , ~of 63Cu gives a 4s unpaired spin population, p4s= 0.62. This is similar to p4s = 0.67, 0.71, 0.69-0.72, and 0.68-0.73 for Cu[C2H4] in Cu[C2H2]in Ar,6 CU[c&6],* and Cu[HCN]l0 in adamantane, respectively. We therefore conclude that 1 is monoethylene copper, Cu[C2H4]The 13C hfi of 16.8 MHz when divided by A = 3777 MHz9 gives p2s = 0.0044. The total unpaired s spin population is therefore -0.62, and the remaining spin population must reside in Cu 4p and/or 3d and C 2p orbitals. The absence of parallel and perpendicular features in the spectra of Cu[C2H4]and Cu[ 13CH2CH2]implies that either the spectra are motionally averaged or that the C 2p orbital contribution is small and the Cu 4p and 3d contributions cancelled each other because the Adio. values are of opposite sign. Species 2. Two copper/ethylene complexes are formed in cyclohexane with small Cu hfi, species 3 with parameters identical with those in adamantane and a species with orthorhombic A and g tensors that are similar to those of Cu[C2H4I2in Ar.6 We conclude that this latter species is indeed diethylene copper, and this is confirmed by the observation of a hfi with two equivalent 13C nuclei in one direction of the spectrum of Cu[l3CH2CH2I2. Species 3. The spectrum given by this species is quite different from that of Cu[C2H4I2in cyclohexane and argon (cf. Figures la and 3a). A possible assignment, by a process of elimination and because of the presence of a 13Chfi, is the 17-electron species Cu[C2H4I3. Evidence in support of this assignment is the fact that the spectrum of 3 has a pattern of lines similar to that of Cu (CO)31 albeit with a small, rat her than zero, perpendicular Cu hfi. Furthermore, a reasonable simulation of the experimental spectrum (Figure 2b) was obtained with the parameters a,,(Cu) = 152.2 MHz, u,(CU) = 60 MHz, aIl(C) = 0 MHz, aL(3 C) = 20 MHz, gll= 2.0021, g, = 2.0120, and P, = 10 MHz. The assignment of the spectrum to triethylene copper is consistent with the axial nature of the A and g tensors. However, a somewhat better simulation (Figure 2c) is obtained with the same parameters but with a perpendicular 13C hyperfine interaction with two equivalent nuclei. Unfortunately, we were unable to unambiguously confirm the number of ligands attached to 3 by the detection of a completely resolved isotropic spectrum as we were with C U ( ' ~ C O in ) ~adamantane." The magnetic parameters of copper/ethylene complexes in inert hydrocarbon and rare-gas matrices are summarized in Table I. A comparison of the Cu hfi of Cu[C2H4]in adamantane and Ar suggests either a fraction more transfer of the unpaired spin to the ligand or slightly more p-orbital character in the SOMO (9) Morton, J. R.; Preston, K. F. J. Mugn. Reson. 1978, 30, 577-582. (10) Howard, J. A.; Sutcliffe, R.; Mile, B. J. Phys. Chem. 1984, 88, 5 155-5 157. (1 1) (a) Howard, J. A.; Mile, B.; Morton, J. R.; Preston, K. F.; Sutcliffe, R. Chem. Phys. Lett. 1985, 117, 115-117. (b) Howard, J. A.; Mile, B.; Morton, J. R.; Preston, K. F.; Sutcliffe, R. J. Phys. Chem. 1986, 90, 1033-1036. (c) Howard, J. A.; Mile, B.; Morton, J. R.; Preston, K. F. J . Phys. Chem. 1986, 90, 2027-2029.
g3
= 2.0010
in the hydrocarbon matrix, although measurable anisotropy does not show up in the powder spectrum. The low value of g (Ag = -0.0093 to -0.0123) is perhaps more consistent with the latter explanation although Ag is slightly smaller in adamantane than in Ar. Values of the Cu hfi of Cu[C2H4I2in Ar and cyclohexane indicate some rotational averaging of the anisotropic parameters in the hydrocarbon matrix that is not frozen out completely even at 5 K. Structural Assignment. Cu[C2H4]. It has been concluded6 that bonding in this complex involves two dative interactions as suggested by Dewar12 and Chatt and Duncanson13 for molecular complexes between olefin molecules and the univalent Cu cation. Thus, there is electron donation from the ethylene ?r-orbital into an empty sp, hydridized orbital on Cu and back-donation of d electron density from the metal 3dY, orbital into the ethylene r*-orbital. The unpaired electron is located in the sp, orbital directed away from the ligand.
1 Our data in adamantane are consistent with this assignment. Theoretical c a l c ~ l a t i o n sthat ~ ~ include electron correlation effects have shown that the ground state of the Cu[C2H4] rcomplex arises from the 3dI04s1atomic configuration of copper. The calculated binding energy is only 0.36 eV and the copper/ ethylene equilibrium distance is 2.37 A. A spin population analysis shows that 99.8% resides in Cu atomic orbitals with 90% in the 4s, 7.5% in the 4p, and 2% in the 4d and only 1% in C 2p orbitals. These results are reasonably consistent with the description of Cu[C2H4]obtained by EPR spectroscopy, considering the inaccuracies involved in estimating s and p unpaired spin populations both theoretically and empirically. The bonding in Cu[C2H4]is however not entirely consistent with the Dewar-Chatt-Duncanson (DCD) model because the C2H4 ligand is not perturbed whereas the DCD model implies a weakening of the r-bond of the ethylene. The bond energy of 0.36 eV is too weak to invoke a charge-transfer bond and is more compatible with a strong van der Waals bond. The alternative structure of Cu[C2H4]has a bonding between Cu and C2H4 to give CuCH2CH2with C,symmetry. This 2A' state is, however, calculated to be 0.5 eV above the 2A' ground state of the Cu[C2H4] r-complex. An earlier SCF-Xa-SW investigation of Cu[C2H415placed the unpaired electron in a 6al orbital with 73% of the electron on Cu and 60% in the 4s orbital. Cu[C2H4I2. Kasai, McLeod, and Watanabe6 suggested that
(12) Dewar, M. J. S. Bull. SOC.Chim. Fr. 1951, 18, C71-79. (13) Chatt, J.; Duncanson, L. A. J. Chem. SOC.1953, 2939-2947. (14) Nicolas, G.; Barthelat, J. C. J. Phys. Chem. 1986, 90, 2870-2877.
Howard et al.
1278 The Journal of Physical Chemistry, Vol. 94, No. 4, 1990
this complex has the planar D2h structure 2 with most of the unpaired spin located in the Cu 4px orbital. Electron flow to the
,*
Z
,
2a 2 ligands can occur from the semioccupied 4px and fully occupied 3d, orbitals to the ligands ?r*-orbitalswith counter electron flow from the ligands' orbitals and the 4py metal orbital. This configuration reduces repulsive overlap between the filled ?r-orbitals and the 4p, orbital. The alternative configuration with a semioccupied 4p, orbital also reduces the repulsive overlap but precludes back-donation of the unpaired electron into the ?r*-orbital. In this context, it is interesting to note that C U ( C O )is~ EPR-silent because the px and pv orbitals are degenerate since they are both coplanar with the T - and ?r*-orbitals of the carbonyl ligands. An alternative structure with the two ligands oriented perpendicular to each other in the point group Dw has been discounted by Kasai, McLeod, and Watanabe.'j The spectrum of Cu[C2H4], in cyclohexane is entirely consistent with the D2hstructure 2. The I3C hyperfine tensor of Cu[C2H4I2is [0, 41.2, 01 and calculation of Adip. from =
+ 2Adip.
(1)
a, = Aiso - Adip.
(2)
all
gives A d i ~ ( ' ~ c=) 13.7 MHz. Dividing this value by CUP= 107.4 MHz9 gives an unpaired 2p spin population on C (p2,) of 0.13 and a total ligand unpaired spin population of -0.5. The total unpaired 2s spin population on the carbon atoms of the ligands (p2, = 0.014) is negligable. This implies an unpaired spin population on the Cu atom of 0.5 and an unpaired spin distribution identical with that of Ag[C,H4].2.i5 The unpaired spin population In the atomic orbitals of Cu can be checked by using the experimental Cu hfi and eq 1 and 2 to determine the isotropic and anisotropic parameters. If all = a ] ; a, = (a2 + a 3 ) / 2 ;and a, is negative, as it must be if a, > all and Adip. is positive then Ai, = -60 MHz and Adip, = 77.2 MHz. Dividing Aiso by A = 5995 MHz9 gives p4s= 4.01. It is difficult to estimate p4 because the best value of P has not yet been established. Tius, we have estimated a value of 171 MHz" that gives p4p = 1.1 2, and Kasai and LindsayI6have recently calculated a value of 21 7.5 MHz that gives p4p = 0.89. Both of these values are too high when compared with the large ligand unpaired spin population. A value of P = 386 MHz is more compatible with the experimental metal hyperfine interactions, a value that is closer to Kasai and Jones'l' original estimate of Aodip.. It is worth noting that Adip. for Cu[C2H4I2is similar to the value of 75 MHz for Cu(CO), that has 37% unpaired spin population in the metal 4p, orbital.Ii Species 3 . The spectrum of 3 produced from 13CH2CH2is consistent with a mononuclear copper complex with either' two or three ethylene ligands. If it has two ligands, it must have a structure different from the DZhstructure 2 because of the difference in the spectra. Kasai, McLeod, and Watanabe considered the D2d structure 2a. In this form, bonding involves electron donation from the ethylene ?r-orbitalsinto the sp,-hybridized orbital and the unpaired electron would have to occupy a degenerate p orbital. This form of Cu[C2H412would not be visible to EPR spectroscopy. (15) Kasai, P. H.; Jones, P. M. J. Am. Chem. SOC.1985, 207,813-818. (16) Lindsay, D. M.; Kasai, P. H. J . Mugn. Reson. 1985, 64, 278-283. (17) Townes, C. H.; Schawlow, A. L. Microwaue Spectroscopy; McGraw-Hill: New York, 1955.
Alternatively, Cu[C2H4I2could have the C, structure 2b. The unpaired electron would however be expected to occupy an sp2-hybridized orbital and have a much larger Cu hfi than is observed for 3.
W
2b If is not the diligand complex, we are left, by a process of elimination, with the trisligand complex, Cu[C2H4I3,although the simulated spectrum of Cu[I3CH2CH2l3is not quite as good as it is for C U [ ~ ~ C H ~ CAH possible ~ ] ~ . structure for Cu[C2H4I3 has the three ethylene ligands aligned with their C=C bonds parallel to the C, axis, Le., a cylindrical-shaped molecule:
3 The SOMO is comprised of the Cu 4p orbital with some ?r*character, Le., back-donation to the three antibonding C2H4 orbitals. The remaining Cu orbitals are sp2 hybridized and accept electron donation from the three filled C2H4 a-orbitals. The alternative structure with the three ethylene ligands coplanar around a central Cu atom, i.e., a planar trigonal molecular with an 2A/ ground state in the point group D3h, is possible because it would give a spectrum with an axially symmetric Cu hyperfine tensor and equivalent 13C nuclei. This structure is however less likely than 3 since there is no r*-p overlap to allow back-donation although the ? r s p overlap can still occur and steric crowding may preclude its formation. The copper isotropic and dipolar terms of 3 are calculated to be 10 and 70 MHz, respectively, if a, is negative as it is for Cu[C2H4I2and C U ( C O ) ~ .Dividing these values by A and CUP gives p4, = 0.01 and p4p= 0.45, assuming P = 386 MHz. These metal unpaired spin populations are similar to those of Cu[C2H412 and indicate little direct contribution from the Cu 4s orbital to the SOMO and a significant 4p orbital contribution. The I3C coupling constant of Cu[C2H4I3could only be estimated by computer simulation and is of rather low precision. The absence of a I3C coupling in the parallel direction and a substantial coupling in the perpendicular direction is indicative of a 2p orbital contribution. Substitution of the values of al,(l3C)and a,(I3C) into eq 1 and 2 gives pZs= 0.0035 and pZp= 0.06 and a total ligand unpaired spin population of -0.4. Interestingly, there is little or no difference between AIlip.(Cu) of Cu [C2H4] and Cu [C2H4I2,suggesting similar unpaired 4p,
J. Phys. Chem. 1990, 94, 1279-1285 and 4p, spin populations. The higher C=C stretching vibration in Cu[C2H4I3of 1517 cm-' compared to 1505 cm"' in C U [ C ~ H J $ ~ either indicates a larger ligand spin population in Cu[C2H4I2,or simply results from a sharing of the same amount of back-donation between three rather than two ethylene ligands. Our attempts to confirm the assignments of infrared bands to copper/ethylene complexes was not successful because samples that gave intense EPR spectra gave very weak IR spectra. The need to include a significant nuclear quadrupole interaction term in the spin Hamiltonians of Cu[C2H412and Cu[C2H4I3is consistent with a large metal porbital contribution to the SOMO that produces an appreciable electric field gradient at the nucleus."
1279
Cu[C2H,], in adamantane decays at 120 K and is markedly less stable than Cu(CO),, which persists to 273 K. Although the energies of the T - and **-orbitals of ethylene are closer to the 4s and 4p orbitals of Cu than the 50- and **-orbitals of CO, this better matching of energy levels is not sufficient to counteract the much better orbital overlap with the more highly directed 5c-orbital and the large amplitude of the **-orbital on the carbon nucleus of CO. Acknowledgment. We thank Drs. J. R. Morton and K. F. Preston (NRCC) and Professor L. Belford for providing the computer programs for spectral analysis and simulation.
Chlorine-35 Nuclear Quadrupole Resonance and Infrared Spectroscopic Studies of Hydrogen Bonding in Complexes of Dichloroacetic Acid with Nitrogen and Oxygen Bases. Correlation of Spectroscopic Properties with Proton Affinity and Aqueous pK, Boleslaw Nogaj,* Institute of Physics, A . Mickiewicz University, 60780 Poznan, Poland
Ewa Dulewicz, Bogumil Brycki, A. Hrynio, Piotr Barczynski, Zofia Dega-Szafran, Miroslaw Szafran,* Department of Chemistry, A . Mickiewicz University, 60780 Poznan, Poland
Piotr Koziol, Institute of Molecular Physics, Polish Academy of Sciences, 601 7 9 Poznan, Poland
and Alan R. Katritzky* Department of Chemistry, University of Florida, Gainesville. Florida 3261 1 (Received: January 3, 1989; In Final Form: July 25, 1989)
Proton affinities (PA), calculated for 57 compounds by the AM1 method, show no general overall correlation with pK, values, but individual classes of bases displayed significant trends. PA and pK, are compared with hydrogen-bonded proton chemical shifts and with centers of gravity of continuous protic infrared absorption: in solution, pyridine complexes and pyridine N-oxide complexes form two distinct families. Their differences in behavior are discussed with respect to energy surface potentials. The 3sClnuclear quadrupole resonance (NQR) frequencies at 77 K were measured for the complexes of 30 bases (with aqueous NQR frequency, tNQR, against pK, values ranging from -10 to 11.25) with dichloroacetic acid. A plot of the average 35CI pK, is shaped similar to that of a titration curve. This dependence is analyzed in terms of the proton-transfer equilibria and can be explained on the basis either of a double-minimum potential function with rapid flipping of the proton between two equilibrium sites or of one broad minimum. The effect of charge localized in the rings of pyridines and pyridine N-oxides on the CI atom is comparable; this could explain why 35CI NQR data of complexes of dichloroacetic acid with various nitrogen against proton affinity (PA) is more scattered, and two and oxygen bases can be treated as one family. The plot of tNqR different curves, one for pyridine complexes and the other for pyridine N-oxide complexes, could be distinguished.
Introduction In previous investigations, we have studied the variations of (i) the chemical shifts of hydrogen-bonded complexes, (ii) the chemical shifts of ISN and I3C, (iii) the center of gravity, (vIR, cm-I), of continuous protic absorption in IR spectra, and (iv) some colligative properties of pyridine complexes and pyridine N-oxide complexes with acetic acids.'+' These data have been correlated (1) Brycki, B.; Szafran, M. J . Chem. SOC.,Perkin Trans. 2 1982, 1333; 1984, 223. (2) Barczynski, P.; Dega-Szafran, Z . ; Szafran, M. J . Chem. SOC.,Perkin Trans. 2 1985, 765. (3) Dega-Szafran, Z . ; Hrynio, A.; Szafran, M. Specrrochim. Acta 1987, 43A, 1553. (4) Dega-Szafran, Z.; Kunzendorf, J. Pol. J . Chem. 1979, 53, 623. (5) Dega-Szafran, Z.; Dulewicz, E. Org. Magn. Res. 1981, 16, 214. (6) Dega-Szafran, Z.; Szafran, M.; Stefaniak, L.; Brevard, C.; Bourdonneau, M. Magn. Res. Chem. 1986, 24, 424.
0022-3654/90/2094-1279$02.50/0
with aqueous pK, values and have demonstrated that pyridines and their N-oxides form two distinct classes of complexes in aprotic solvents. However, in the solid state, a plot of 3SClnuclear quadrupole resonance (NQR) mean frequency shifts against aqueous ApK, values is similar in shape to a titration curve, and complexes of both pyridines and their N-oxides fall on the same curve. We now extend our N Q R investigations to complexes of dichloroacetic acid. Using the AM1 method, we have calculated gas-phase proton affinities (PA) of bases. We examine correlations of the previous N M R and IR data with PA and compare them (7) Brycki, B.; Nowak-Wydra, B.; Szafran, M. Mag. Res. Chem. 1988,26, 303. (8) Deaa-Szafran. 2.;Szafran, M.; Krealewski, M. J . Chem. SOC.,Perkin Trans. z i980, I 5 I 6. (9) Nogaj, B.; Brycki. B.; Dega-Szafran, Z.; Szafran, M.; Mackowiak, M. J . Chem. Soc., Faraday Trans. 1 1987,83, 2541.
0 1990 American Chemical Society
