1384 of 30% enriched ISN03Nacontained in a coaxial 2-mm tube centered in a IO-" sample tube. The solvent effect on 6I5N did not exceed -0.8 ppm upfield when going from pure liquid ( D M F (1)) to a 30% solution in CHC13, CHC12CHC12, or CH3CN. Fifty percent solutions of D M F in methanol or ethanol cause more important downfield shifts 9 2 to 3 ppm. The influence of Cr(AcAc)3 on 6I5N was checked by studying the I5N resonance of enriched materials without using T I reagent. The add i ~ o nof 0.1 M Cr(AcAc)3 to D M F and DMA (2) brings about a small upfield shift (=1.5 ppm) for 615N. Materials. Commercially produced amides, thioamides, and carbamoyl derivatives were carefully dried and distilled (1-4,lO-13,20, 21,24). The iminium salts and acceptor-donor complexes (14-19) were synthetized as previously de~cribed.~' Vinylamides (6,7) and enamines (8, 9, 22, 23) were prepared according to conventional
procedure^.^ I .4 I .42 Labeling experiments: 5% I5N enriched DMA was synthetized by the reaction of acetyl chloride with a cold solution of 5% I5N dimethylamine in hexamethylphosphoramide; 5% sN D M F was prepared from the reaction of CC13CHO with enriched dimethylamine dissolved in CHC13 (40% yield).44 Five percent I5N dimethylamine hydrochloride was prepared from 5% I5N ammonium chloride (CEA, Paris) and a 30% aqueous solution of f ~ r m a l d e h y d e A . ~solution ~ of 5% I5N (CH3)2NH?CI in hot water is poured into 10 N NaOH and the gaseous enriched dimethylamine is collected in the appropriate solvent.
Acknowledgments. We thank Dr. S. Odiot for performing CNDO/2 calculations and the C.N.R.S. for financial support. References and Notes P. Hampson and A. Mathias, Mol. Phys., 11, 541 (1966); 13, 361
(1967). T. H. Siddall 111, W. E. Stewart, and F. D. Knight, J. Phys. Chem., 74,3580
(1970). P. A. Kollman, W. F. Tragar, S. Rothenberg, and J. E. Williams, J. Am. Chem. SOC., 95, 458 (1973). E. S. Gore, D. J. Blears, and S. S. Danyluck, Can. J. Chem., 43, 2135
(1965). B. M. Rode and R. Fussenegger, Trans. Faraday SOC., 1958 (1975). G. Olofsson, P. Stilbs, T. makenberg, and S. Forsen, Te!raMron, 27, 4583
(1971). J. S. Hartman and R. R. Yetman, Can. J. Chem., 54, 1130 (1976). E. L. Eliel, "The Stereochemistry of Carbon Compounds", McGraw-Hill,
New York, N.Y., 1962, p 341. (9) H. Kessler. Angew. Chem., 9, 219 (1970). (IO) G. J. Martin and S. Poignant, J. Chem. SOC., Perkin Trans. 2, 642 (1974). (1 1) G. J. Martin and J. P. Gouesnard, Tetrahedron Lett., 4251 (1975). (12) M. Karplus and J. A. Pople, J. Chem. Phys., 38, 2803 (1963). (13) G. A. Webb and M. Witanowski, "Nitrogen NMR". M. Witanowski and G. A. Webb, Ed.. Plenum Press, New York, N.Y., 1973, pp 1-39. (14) T. Tokuhiro and G. Fraenkel, J. Am. Chem. SOC., 91, 5005 (1969). (15) G. W. Mines and H. W. Thompson, Spectrochim. Acta, Part A, 31a, 137 (1975). (16) G. E. Maciel and D. A. Beatty, J. Phys. Chem., 69, 3920 (1965). (17) M. J. Janssen, Recl. Trav. Chim. Pays-Bas, 79, 454 (1960). (18) T. Drakenberg. K. J. Dahlquist. and S. Forsen, J. Phys. Chem., 76, 2178 (1972). (19) G. J. Martin, M. Berry, D. Le Botlan. and B. Mechin, J. Magn. Reson., 23, 523 (1976). (20) K. Rabinovitz and A. Pines, J. Am. Chem. Soc.,91, 1558 (1969). (21) M. L. Martin, G. Ricolleau, S. Poigant, and G. J. Martin, J. Chem. Soc., Perkin Trans. 2, 182 (1976). (22) R . C. Neuman and V. Jonas, J. Org. Chem., 39,929 (1974). (23) R. C. Neuman and V. Jonas, J. Am. Chem. SOC., 90, 1970 (1968). (24) L. W. Reeves, R. C. Shaddick, and K. N. Shaw, Can.' J. Chem., 49,3683 (1971). (25) L. M. Jackman, T. E. Kavanagh, and R. C. Haddon, Urg. Magn. Reson., 1, 109 (1969). (26) E. A. Allan, R. F. Hobson, L. W. Reeves, and K. N. Shaw, J. Am. Chem. Soc., 94, 6604 (1972). (27) R . C. Newman, D. N. Roark, and V. Jonas, J. Am. Chem. SOC., 89,3412 (1967). (28) T. Drakenberg, K. I. Dahlquist, and S. Forsen, Acta Chem. Scad., 24,694 (1970). (29) L. Isbrandt, W. C. T. Tung, and M. Rogers, J. Magn. Reson., 9, 461 (1973). (30) V. S. Dimitrov, J. Magn. Reson., 22, 71 (1976). (31) M. L. Blanchard, A. Chevallier, and G. J. Martin, Tetrahedron Lett., 5057 (1967). (32) M. L. Filleux, N. Naulet, J. P. Dorie, G. J. Martin, J. Pornet, and L. Miginiac, Tetrahedron Lett., 1435 (1974). (33) T. Drakenberg, J. Phys. Chem., 80, 1023 (1976). (34) J. Sandstrom, J. Phys. Chem., 71, 2318 (1967). (35) R. F. Hobson, L. W. Reeves, and K. N. Shaw, J. Phys. Chem., 77, 1228 (1973). (36) F. M. Menger and G. Saito, J. Urg. Chem.. 40, 2003 (1975). (37) CI. Rabiller, J. P.Renou. and G. J. Martin, J. Chem. Soc., Perkin Trans. 2, in press.
(38) A. Calzolari, F. Conti, and C. Franconi, J. Chem. SOC.6,555 (1970). (39) R. Merenyi, private communication. (40) M. L. Filleux. Org. Magn. Reson., in press. (41) D. L. Muck and E. R. Wilson, J. Org. Chem., 33, 419 (1968). (42) R. Dulou, E. Elkik, and A. Veillard, Bull. SOC.Chim. Fr., 967 (1960). 143) A. I. Vopel, "Pratical Organic Chemistry", 3d ed, Longmans, New York, N.Y., 19-56. p 416. (44) F. F. Blicke and C-J. Lu. J. Am. Chem. SOC., 74, 3933 (1952) (45) H. A. Scheraga, Adv. Phys. Urg. Chem., 6, 103 (1968).
Nitrogen- 14 Nuclear Quadrupole Resonance Spectra of Coordinated Pyridine' Yu-Nian Hsieh? Gerald V. Rubenacker, C. P. Cheng, and Theodore L. Brown*3 Contribution from the School of Chemical Sciences and Materials Research Laboratory, University of Illinois, Urbana, Illinois 61801. Received May 13, 1976
Abstract: Nitrogen-14 nuclear quadrupole resonance (NQR) data have been obtained for several pyridine complexes with Zn(I1) and Cd(II), I(CsHsN)2C104, Br(CsHsN)zC104, HNCsH5NO3, and 4-CH3C5HdNO. The field gradient tensor is substantially altered upon coordination to a Lewis acid. Using a modification of the Townes-Dailey model it is possible to relate the field gradient parameters to the extent of charge transfer from nitrogen to the Lewis acid. I4N NQR signals were also detected for the nitrato nitrogens in several cases and for the NCS- ligand in Zn(NCS)2(CsHsN)2. The NQR data for this complex are consistent with coordination to Zn(I1) through N .
During the past several years the number of reported I4N nuclear quadrupole resonance (NQR) spectra has grown rapidly. Nevertheless, the number and variety of data available for compounds in which a nitrogen atom is coordinated directly to a metal ion or other Lewis acid is quite limited. We have indicated in another contribution the reasons why I4N N Q R spectra should be of interest in such systems4 The adiabatic Journal of the American Chemical Society
/
99:5
demagnetization in the lab frame, double resonance levelcrossing technique ( d r l ~ ) , is~ of - ~particular value in obtaining the desired data, because it is sensitive and capable of detecting resonances at frequencies too low for direct detection. The advantages and limitations of the technique have been described by E d m o n d ~ . ~ . ~ In the present contribution we report the I4N NQR spectra
/ March 2, 1977
1385 for several adducts of pyridine with various Lewis acids. The compounds studied include several zinc(I1) complexes and two complexes of Cd(I1). In addition, we have observed the I4N NQR spectrum of the N-bonded coordinated thiocyanate and of nitrate in four different substances.
Experimental Section Sample Preparations. Dichlorobispyridinezinc(II) (ZnCI*(py)z), ZnBr2(py)z, and ZnIz(py)z were prepared by precipitation from an ethanol solution of zinc halide by the addition of pyridine.1° Zn(NCS)z(py)* was prepared similarly from aqueous solution.’ I The pyridinium salts were obtained by reaction of equimolar amounts of concentrated acid and pyridine. All were recrystallized from ethanol. -Bispyridinesilver(I) perchlorate, Ag(py)zCI04, precipitates when an aqueous AgC104 solution is mixed with an aqueous pyridine solution.I2 It was dried under vacuum in the dark. From this, bispyridinebromine( I) perchlorate, Br(py)zCIO4, was prepared by a published procedure.” Bispyridineiodine(1)perchlorate, I(py)zC104, was prepared similarly, except that Iz was added as a solid. The product is fairly insoluble in a pyridine-chloroform mixture and was separated from AgI by dissolving it in methanol. The two bisnitratozinc(I1) complexes, Zn(N03)2(py)2 and Zn(N03)2(py)3, were prepared by the method of O ~ e l l e t t e ,using ’~ AgNO3 and Zn as starting materials. All sample preparations yielded satisfactory elemental analyses for C, H, and N . Dichlorobispyridinecadmium(II), CdC12(py)2, was prepared by precipitation from an aqueous solution of CdCl2 using a large excess of pyridine. Anal. Calcd for CdC12CloN2Hlo: C, 35.2; N , 8.23; H , 2.95. Found: C, 36.2; N, 8.39; H, 3.08. (Since the compound might readily lose pyridine it was prepared and handled in the presence of excess pyridine, to prevent decomposition. Thus, the solid probably contains a slight excess of pyridine, but this will not affect the NQR results.) Dinitratotrispyridinecadmium(lI), Cd(NO3)2(py)3, was prepared by the method of 0 ~ e l l e t t e . l ~ NQR Measurements. The I4N NQR measurements were carried out in part as described p r e v i ~ u s l ysample ;~ temperature in all cases was 77 K. Values of proton T I ranged from 8 s in Zn(NO3)z(py)3 to 900 s in (pyH)(NO3). Approximate values of T2 were in the range from 6 to 14 f i s . During the course of this work the instrument was modified to permit single-coil as opposed to cross-coil operation. Receiver recovery time was shortened by use of active filtering. This modification has resulted in shorter overall recovery time and consequently increased sensitivities. Table I lists the observed I4N transition frequencies for a variety of compounds in which the pyridine is coordinated to a Lewis acid. For all nitrate-containing substances transitions associated with the nitrate group were also observed. Similarly, the transitions of the nitrogen of the NCS- ligand were seen in Zn(NCS)z(py)z. In all instances the transitions were actually observed, rather than calculated from other observed frequencies. The precision with which the frequencies are determined varies, but in general they are measured to within 2 to 5 kHz.
Discussion Comparisons with Crystal Structure Data. For several of the compounds listed, x-ray crystal structure determinations have been carried out. The number of nonequivalent I4N sites is therefore known for the samples at room temperature. The expected number of lines assigned to pyridine is seen for Z ~ C M P Y )’ CdCMpy)2,l6 ~,~~ (pyH)(N03),I7 Zn(NO3h(PY)2,’8Zn(N03)2(PY)3,19and Cd(N03)2(PY)3.20 Interpretation of Data for Coordinated Pyridine. The most obvious message of the pyridine data in Table I is that coordination of the Lewis acid to the lone pair of the pyridine nitrogen causes a substantial decrease in field gradient. This suggests that, aside from whatever detailed interpretations may be placed upon the data, the NQR technique will prove to be a sensitive probe of bond formation at nitrogen. Hopefully the NQR data will be useful in providing information on the nature of the bond from nitrogen to the Lewis acid, for example, as regards covalency. To extract such interpretive detail, however, Brown et al.
Table I. Measured I4N Quadrupole Resonance Transitions (kHz) in Pyridine ComDounds at 77 K Compd
v+
Y-
3892 2387 2332 2345 2282 2360 2321 2297 760 715 2124 2097 572 2578 2569 547 2850 2718 2582 545 955 584 1330 1398 3688 I171
2984 2078 2038 2040 2000 2053 2029 2035 660 615 1884 1877 288 2180 2201 309 2298 2272 2179 350 678 445 1210 725 2933 586
VQ
908 309 294 305 282 307 292 262 100 100 240 220 284 398 368 238 552 446 403 195 277 120 673 756 585
e2qQlh 4584 2977 2913 2923 2855 2942 2900 2888 947 887 2672 2649 573 3172 3180 571 3432 3327 3174 597 1090 685 1693 1415 4414 1171
7
0.396 0.207 0.202 0.209 0.198 0.208 0.201 0.181 0.211 0.224 0.180 0.166 0.991 0.251 0.231 0.834 0.320 0.268 0.254 0.654 0.509 0.405 0.142 0.951 0.342 0.999
a The pyridine values are an average of four data sets for crystallographically nonequivalent molecules.b L. Guibe, Ann. Phys. (Paris), 7, 177 (1961).
it is necessary to apply some kind of model to the results. For pyridine several ab i n i t i ~ ~asl -well ~ ~ as more empirica125,26molecular orbital calculations have been carried out. Comparison of calculated and observed values for e2Qq is made more tenuous by the fact that a value of e2Qqofor atomic nitrogen is not accurately known. Typically, the values of e2Qq obtained from experimental data are in reasonable agreement with calculated values using the current best estimate for Q . On the other hand, the calculated asymmetry parameter, q, is generally not in good agreement. For example, the recent ab initio calculation for pyridine by Ha and O ’ K ~ n s kyielded i~~ an q of 0.14, as compared with an observed value of 0.39. More empirical molecular orbital approaches suffer from the necessity to fix values for quite a large number of factors that might affect the field gradient tensor. In addition the field gardient operator itself may be only incompletely evaluated from the basis set of occupied M O ’ S . ~ ~ , ~ ~ To understand the changes at nitrogen which accompany Lewis acid-base adduct formation on the basis of a theoretical model, it would appear best to parameterize the model in terms of pyridine as a reference compound. For the present we have elected to employ a modified version of the Townes-Dailey as a guide. In this model the field gradient at I4N is expressed entirely in terms of the populations of the 2p orbitals at nitrogen. The populations are deduced from the experimental NQR results or assigned empirically. In applying the Townes-Dailey model to pyridine, the axis system shown in Figure 1 is adopted. Assume that the CNC angle is 26 and that the nitrogen orbitals within the plane of the molecule are hybridized so that they are directed along the N-C bond axes. The occupancy of the nitrogen u orbital directed toward carbon is assigned a value bo and that of the pT orbital a value ao. The lone-pair orbital is assumed to have an occupancy of 2. The diagonal components of the field gradient tensor then have values
/ 14N Nuclear Quadrupole Resonance Spectra of Coordinated Pyridine
1386
Figure 1. Coordinate system and orbitals involved in pyridine coordination.
- a o / 2 + bo(cot2 e - Ih)]qo - cot2 e) + - bo(1 + cot2 6 ) / 2 ] q o - Cot2 e) - a o / 2 + bo(2 - cot2 e)iqo
qzz= [2(1 - cot2 e)
qvv = [ - ( 1 qxx = [-(I
a0
(1)
u = uo
+ A ( 2 - a)
b = bo
+ B(2 - a)
-
(2)
(3) That is, the populations of the a and N-C u orbitals increase in proportion to the degree of electron withdrawal from nitrogen, with proportionality constants A and B, respectively. We assume in addition that there is no change in the geometry at nitrogen upon coordination and that the only mode of interaction between pyridine and the acid is via the u bond from nitrogen. Then, in general a = (1 - cot2 0)u +
+ A(2 - a)] + (Cot2 0 - '/2)[bo + B(2 - u)]
'/*[a0
+
a7 = 3/2 {[bo B(2
- a)] - [ U O + A(2 - o ) ] )
(4) (5)
By eliminating u in these two expressions one obtains a linear relationship between 1 / a and 7:
K=
[ 2 ( B- A )
1 / a = 1/ K + ( L / K ) q (6 ) + (bo - U O ) + (Ab0 - Bao)](l - Cot2 0)
(B - A )
(7) 2 [ ( 1 - Cot2 19) A / 2 - B(cot2 0 - '/2)] L= (8) 3(B - A ) These expressions are valid in the range of u values such that the major axis of the field gradient tensor remains oriented along the donor orbital direction. The linear relationship described by eq 6 is constrained to pass through the pyridine data, for which both a and 7 are known, and for which a is 2. As a
+
Journal of the American Chemical Society
/
99:5
I
0 25
I
1
0 30
0 35
J
0 40
7
The quadrupole coupling constant e2Qqz2is then given by a(e2Qq0)where a = [ 2 ( 1 - cot2 e) - ao/2 bo(cot2 e $91. The asymmetry parameter 7 is given by r l = (qxx - qvv)/q2z. Manipulation of eq 1 yields an expression for 7. Since both e2Qqzzand 7 are measured for pyridine, both a0 and bo can be calculated from the data after choosing values for 28 and e'Qq0. When pyridine is bound to a Lewis acid the formerly lone pair orbital becomes the donor orbital. The donor orbital occupancy is reduced in the adduct from 2 in free pyridine to some value u. It is, of course, this occupancy of the donor orbital that is of major interest. To estimate u from the value of e2QqzZand 7) for the coordinated pyridine, it is necessary to make assumptions about the effect of coordination on the occupancies of the other orbitals. One possibility, for example, would be to assume that no changes occur in either a0 or bo. However, donation of the lone pair electrons on nitrogen to a Lewis acid will cause some increase in the occupancies of the other three orbitals via an inductive effect. We assume that the occupancies of the nitrogen orbitals in the pyridine adducts are given by
+
1
0 20
Figure 2. Graph of h/e2QqZ,vs. 7 for Zn(I1) and Cd(I1) complexes of pyridine. The point labeled 1 represents pyridine.
result, eq 7 and 8 are linearly dependent; A and B are thus linearly related unknowns. From the experimental data for an appropriate series of pyridine adducts the linear relationship between 1/ a and 7 predicted by eq 6 can be tested. In the series of zinc(I1) and cadmium(I1) complexes the extent of electron transfer from nitrogen to the metal is expected to be in the range of a few tenths of an electronic charge and to vary over a relatively narrow range throughout the series. Accordingly, we have employed these data to test eq 6. It is convenient to graph h/ e2Qqzzvs. 7, because 1 / a = e2Qq0/e2Qq2,;values for K and L can then be determined from the graph by simply multiplying slope and intercept by the assumed value for e2Qqo/
h. The graphs of h/e2QqZZ vs. 7 for all the data for the zinc and cadmium complexes and for pyridine itself are shown in Figure 2. The line is constrained to pass through the pyridine datum. The data do indeed appear to define a reasonably good linear relationship. As an illustration of the application of the relationship we assume that e2Qq0/h is 9.0 MHz; and 20 = 120'; this yields values of a0 = 1.203 and bo = 1.337 for free pyridine. Using these values and the parameters of the line depicted in Figure 2 we obtain the relationship B = -0.299 0.710A. If the model chosen is to be physically reasonable, A and B are subject to certain constraints. Both A and B should be positive; withdrawal of the lone pair electrons from nitrogen by a Lewis acid should result in an increase in the orbital occupancies of both the a and the other two u orbitals. On the other hand, upper bounds for A and B are determined by the assumption that A 2B should be less than 1 . That is, the total increase in populations of the other orbitals on nitrogen should be smaller than the loss in population in the donor orbital. Applied to the linear relationship between A and B indicated above, these considerations suggest that 0 < B < 0.17 and 0.42 < A < 0.66;thus, the allowable range for these parameters is not large. It seems reasonable to choose values in the middle of these ranges: B = 0.08,A = 0.50.Thus 2B A = 0.66;that is, about 66% of the charge withdrawn from nitrogen via the acid-base interaction is replaced by inductive electron shifts in the u and ?r bonds. It is noteworthy that most of this shift occurs in the more polarizable a electron system, a result consistent with molecular orbital calculations (vide infra). The evaluation of parameters can be carried through for other choices of e2Qqo. This causes changes in the values of a0 and bo but not in the linear relationship between A and B . Similarly, a hybridization other than pure sp2 could be assumed. The observed CNC angle in pyridine is 1 17°.28 When hybridization consistent with this angle is assumed, the re-
/ March 2, 1977
+
+
+
1387 Table 11. I4N Orbital Populations in Pyridine
eZQqo.M H z
C N C angle, deg
a0
bo
8.4 8.4 9.0 9.0 10.0 10.0
120 117 120 117 120 117
1.145 1.097 1.203 1.156 1.282 1.240
1.290 1.241 1.337 1.290 1.403 1.361
Table 111. Pyridine Donor Orbital Occupancies ComDd
U
PY 2.00 CdCIz(py)z 1.87 f 0.02 C d ( N O h ( p y ) 3 1.842 f 0.001 1.826 f 0.003 Zn(N03)2(py)3 1.826 f 0.002 1.817 f 0.006 Zn(N03)2(py)2 1.767 f 0.007 1.761 f 0.004 Z ~ M P Y ) ~ 1.793 f 0.001 1.788 f 0.001
ComDd
4
Z ~ B ~ A P Y ) ~1.792 f 0.00 1 1.781 f 0.002 1.795 f 0.003 ZnCIz(py)2 1.789 f 0.001 Zn(NCS)2(py)2 1.78 I f 0.006 1.640 f 0.004 I(py)2C104 1.57 f 0.03 Br(py)2C104 HpyNO3 1.38 f 0.01
+
sultant equation between A and B is B = -0.293 0.707A. Table I1 summarizes the values of a0 and bo calculated for various choices of CNC angle and e2Qqo. To provide a basis for testing and applying the model it seems best to adopt a reasonable set of structural and model parameters and then to assume these fixed for all compounds studied. We adopt a CNC angle of 117O, the angle observed in pyridine. There is no evidence from the relatively imprecise crystallographic data for a marked change in this angle upon coordination of pyridine. We assume it remains at 117' in the pyridine adducts. For e2Qqowe chose 9.0 MHz, a value in the midrange of those employed by various worker^.*^*^^ The effects of a variation in this quantity on the results extracted from the model are discussed below. We further assume that A = 0.50, which yields B = 0.06, so that A 2B = 0.62. With these choices, it is possible to calculate e2Qq and 7 as a function of u, that is, as a function of the population of the nitrogen donor orbital. Graphs of e2Qq and 7 vs. u are shown in Figure 3. These graphs provide the connection between the NQR data and chemically interesting insights into the interaction between nitrogen and the Lewis acid. For any given compound, given the values of e2Qq and 7,one should be able to locate a place along these curves a t which both data fit on their respective curves for the same value of u. However, there are many factors that might cause either e2Qq or 7 to deviate from the expected relationship. Most notably, interactions of the pyridine ring with atoms and ions of the solid lattice other than the one to which the pyridine ring is coordinated could cause distortions of the field gradient tensor. It is not obvious that such interactions would affect one of the two field gradient parameters more than the other. It is evident from the graph that for values of CT in the range 1.6 to 1.4 the dependence of 7 on u is very sharp. One might expect relatively poor fits of both e2Qq and 7 to the graphs in this region. However, the question of importance is how much uncertainty in u is involved in fitting the data for a given nitrogen onto the graph. Because the data for the zinc complexes and for Cd(N03)2(py)3 all fell very close to both curves, the data for these compounds are not shown. The pairs of points that are shown, in the order of decreasing values of u, represent CdClz(py)z, I(py)2C104, Br(py)2C104, and HpyN03, respectively. Table I11 lists the values of u estimated from fittings in Figure 3 for all the compounds studied. The uncertainties assigned to these
+
Donor Orbital Occupation
Figure 3. Quadruple coupling constant, e2Qq/h (KHz) (dashed line), and asymmetry parameter T vs. nitrogen donor orbital population in coordinated pyridine ( A = 0.50, B = 0.06, e2Qqo = 9.0 MHz, LCNC = 1 17"). The values of e2Qqo and q calculated from the data for the Zn and Cd complexes other than CdCl2(py)z lie on the lines and are not shown. The compounds for which the data are shown are: ( 1 ) CdCl~(py)?;(2) I(py)2CIO4; (3) Br(py)zCIO4; (4) HpyN03. For each compound the corresponding values of e2Qq/h and 7, connected by a dotted line, are placed along the horizontal scale to obtain the best match of each quantity to the calculated curves.
values are estimates of the uncertainties in fitting the data to the graph. The nature of the curves for e2Qq and 7 as a function of u is such that in certain ranges of u there is the possibility of some ambiguity in the placement of the data. For example, the data points for I(py)2C104 might have been placed on the graph at a much lower value for u, around 1.3, albeit with poorer fit. The value for Br(py)2C104 could similarly have been fitted (though not as well) at lower CT. The points for HpyN03, on the other hand, could be fit as well a t about 1.53 as at 1.38. However, the fits we have chosen can be defended as decidedly the best ones in terms of mimimizing differences between calculated and observed values of e2Qq and 7 and on grounds of chemical reasonableness. For a given charge on the Lewis acid and a given coordination number, the value of u should increase with increasing radius of the Lewis acid. Thus, the values of u for the cadmium complexes are higher than for analogous zinc species. Similarly, the value of u for I(py)2C104 should be larger than for Br(py)zClOd, as it is in our placement. Finally, the value of u for HypNOs should be substantially lower than for the other two monopositive ions examined. It is perhaps worth pointing out that the points corresponding to values of 7 of 0 or 1 correspond to points a t which the relative values of field gradient components along the molecular axes are changed. Thus, for example, a t u = 1.68 the x and y components are equal in magnitude. At u = 1.53 the major axis direction is shifted to the direction normal to the plane of the molecule. Significance of Derived Values of Donor Orbital Population. The model which leads to the results shown in Figure 3 and Table I11 may fail to give a precise description of the orbital populations a t coordinated nitrogen for several reasons: (a) The geometry about nitrogen might change upon adduct formation, contrary to the assumption of the model. So long as the bonding around nitrogen remains planar, small bond angle changes proportional to the energy of acid-base interactions are absorbed in the model in the form of assumptions about the 0 orbital occupancies. In any event, the extant crystal structure data do not provide sufficiently accurate data on the geometry about the nitrogen in coordinated pyridine to reveal
Brown et al. / I4N Nuclear Quadrupole Resonance Spectra of Coordinated Pyridine
1388 Table IV. Orbital Populations from Molecular Orbital Calculations of NCsH5 and HNCsHs+ 35
Mulliken population Nitroeen orbital
NC
