J. Phys. Chem. 1989, 93,1265-1269
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Figure 9. Relative intensity increase of the single observable peak in the
higher molecular weight polyethylenes.
C-C stretch vibrational coordinates. The intermolecular coupling arises from the compression of adjacent molecules and is a direct result of pressure. The resonance is apparently assisted by the decrease in energy separation of the two modes, which in turn arises from the inherent shift behavior of the polyethylenes. The primary effect of pressure is to increase the intensity of the CH2 wagging mode via intermolecular coupling. At sufficiently high pressures, the second-order effect of phonon-assisted resonance becomes an increasingly dominant factor in the behavior of the two vibrational modes studied. This is best demonstrated in the polyethylene M , = IO0 where the phonon-assisted resonance
Zinc Complexes of Water, Hydroxide, and Ammonia Douglas B. Kitchen and L. C. Allen* Department of Chemistry, Princeton University, Princeton, New Jersey 08544 (Received: March 9, 1987)
Several complexes of chemical and biochemical interest were investigated by ab initio MO calculations using effective core potentials. The binding energies of water and hydroxide ion to (NH3)3Zn2+were determined to be 43 and 284 kcal/mol, respectively. The proton affinity of the hydroxide complex was determined to be 167 kcal/mol. The first two results are likely to be in error by less than 3% while the proton affinity may be in error by as muchas 20 kcal/mol. The effective core potentials of Hay and Wadt (J. Chem. Phys. 1985,82, 270) and Stephens, Basch, and Krauss (J. Chem. Phys. 1984, 81, 6026) have been tested against experimental results for some small zinc and copper systems and were found to be very accurate. Correlation effects on the reaction energies of closed-shell reactions were found to be in the range of 2-3% versus 50% for reactions to open-shell products. The appropriate degree of contraction for the zinc basis set that would provide accurate results and computational efficiency for larger systems is a 2s,lp,ld contraction on zinc with a 31G contraction on 0, N, and H. Added p and d functions were found to be unnecessary for the charge-transfer systems.
Introduction With the advent of effective core potentials (ECP's) for most of the periodic table, new possibilities exist for the study of transition-metal compounds using a b initio MO methods. Zinc chemistry is one area of great importance because of its ability to form alloys and amalgam' and because its occurrence in biology is second only to that of iron among transition elements. More than 80 enzymes containing zinc have been reported, the best known of which are carboxypeptidase A, liver alcohol de-
* To whom correspondence should be addressed.
hydrogenase, and carbonic anhydrase.* In most of these, zinc coordinates to water, hydroxide, or amines, thus providing a major motivation for the study reported here. We have investigated the Hay and Wadt ECP's3 (H&W) for zinc and the Stephens, Basch, (1) Cotton, F. A.; Wilkinson, G. Advanced Inorganic Chemistry; Wiley: New York, 1980; p 589. (2) (a) Cook, C. M.; Haydock, K.;Lee,R. H.; Allen, L. C. J . Phys. Chem. 1984,88,4875. (b) Cook, C. M.;Allen, L. C. Ann. N.Y.Acad. Sci. 1984, 429, 84. (c) Cook, C. M.; Lee, R. H.; Allen, L. C. In?. J . Quantum Chem. Symp. 1983, 10, 263. (3) Hay, P. J.; Wadt, W. R. J . Chem. Phys. 1985, 82, 270.
0022-3654/89/2093-1265$01.50/00 1989 American Chemical Society
7266 The Journal of Physical Chemistry, Vol. 93, No. 20, 1989
Kitchen and Allen
TABLE I: Geometries and Proton Affinities: O K and H@" r(0-H)
OHPA
31G;31G 31G*:3 1G 121G;311G 211G;311G
0.978 0.962 0.974 0.974
total energy r(0-H) ECP Hartree-Fock Resultsb 408.8 -16.17584 0.956 -16.18844 0.952 415.7 408.8 -16.178 60 0.951 408.6 -16.17992 0.953
6-31G*' 6-3 1 l+G** Lee et aLd P&D'
0.962 0.945 0.943 0.98
All-Electron Hartree-Fock Results 429.3 -75.326 60 0.947 406.4 -75.405 66 0.941 407. -75.416 35 0.941 319.4
HF MP2 MP3 MP4(SDTQ) exptf
All-Electron Results at MP4(SDTQ)/6-31 I++G** Geometries 406.2 -75.405 17 398.3 -75.640 20 405.7 -75.630 98 0.969 399.8 -75.650 12 0.96 1 0.97 3989 0.957
H20 Zn2++ H20. *Reference9. 'Assumed bond length.
-
and water to carbonic anhydrase and other less well characterized enzymes. These calculations utilized zinc contractions of 2s,lp,ld and 2s,lp,2d and the ligands represented by the 31G basis. The geometry was optimized with the geometrical restriction that all NH and O H bond lengths were identical. As found for the small complexes of Table IV, the results for both representations were the same. Thus we are finding that rules for zinc contractions and ligand basis sets can be established on the simplest systems and extrapolated to large complexes of chemical and biochemical interest. In Tables I, IV and V are entries from the pioneering work of h l m a n and Demoulin, who used an earlier set of ECP's and were the first to systematically study zinc complexes. Because of the lack of experimental data and other calculations now available, it is not surprising to find some inconsistencies in their results.
In each of the systems they studied (OH-, H 2 0 , Zn(NH3)2+, Zn(OH)+, ZXI(OH~)~+, (NH3)3Zn(OH)+,and (NH3)3Zn(OH2)2+) the internuclear distances were well reproduced but binding energies and proton affinities are considerably in error, the most severe of which is an almost 2 times too large estimate of the binding energy of water to (NH3)3Zn2+. They represented the zinc valence electrons by 4p orbitals and 3d orbitals only: the 4s orbital was included by the addition of the sixth d function. Due to the less diffuse nature of the d orbitals, the 4s function was not well represented, and from our study this proves to be the most important feature of the basis set. To determine if the choice of effective potential may have biased the results, the ZnOH+ and ZnOH?+ calculation were performed with the H & W and SB&K ECP's and the Pullman and Demoulin valence basis set as given in ref 8. Within a few kilocalories per mole the binding
J . Phys. Chem. 1989, 93, 7269-7275 TABLE VI: Comparison of Core Representations'
BE Zn(OH2)2+ BE Zn(OH)+ PA Zn(OH)+ PA (OH)-
P&D all electronb
H&W ECP, P&D basis setc
112.4 46 1 81.7 44 1
119.5 460 90.2 441.9
"All results in kilocalories per mole. bFrom ref 8. CECP from H&W and valence basis set from ref 8. energy and proton affinities were reproduced (see Table VI). This eliminates the ECP's as the source of error. The results obtained for the large complexes (Table V) indicate that water and hydroxide ion bind to (NH3)3Zn2+by 43 and 284 kcal/mol, respectively (see Table V, footnote a for the definition of binding energies). These results do not include electron correlation, but the results on the smaller complexes suggest that the increase in binding due to electron correlation is only 2-3%. Zero-point-energy corrections (ZPE) are also of the same magnitude but opposite direction. The proton affinity of the hydroxide complex is calculated to be 167 kcal/mol. Since the proton affinity is also equal to the difference in binding of hydroxide and water subtracted from the proton affinity of the hydroxide ion, the correlation correction in the latter will have an effect on the correlation correction of the proton affintity of the complexed hydroxide ion. Assuming a 2.5% increase in the binding energies due to the correlation correction and an 8 kcal/mol decrease in the hydroxide ion proton affinity, the net correlation correction for the proton affinity of the complexed hydroxide is a decrease of 14 kcal/mol. ZPE corrections would also decrease the proton affinity, but the correction would be less than the correction for the hydroxide proton affinity (8 kcal/mol) since the ZPE corrections for the binding of hydroxide and water are likely to be the same sign but subtracted from the hydroxide correction. We therefore believe that the estimates found in Table V of the binding energies of these large complexes are indeed good estimates for
7269
these properties and are not likely to need future modification. However, the proton affinity may be as much as 14-22 kcaljmol too large.
Conclusions (1) The H & W and SB&K ECP's and basis sets have been shown to be sufficient for obtaining reliable bond energy results for zinc complexes when double {contractions are used. (2) The effect of electron correlation on reaction energies for Zn and Cu is small on clased-shell reactions (typically the Hartree-Fock level is only 2% too low) while it is very si&icant on reactions involving open-shell species (typically the Hartree-Fock level is 50% too low). (3) The effects of the contraction of the basis sets reveal that minimal basis sets should give reasonable geometries but poor reaction energetics for closed-shell systems. The most important consideration in the choice of basis set is that there should be a flexible representation of the 4s orbitals (but not necessarily more than a minimal representation of the 4p and 3d orbitals). d polarization functions on oxygen have a significant largely predictable effect on the bond angle around the oxygen but not on bond lengths or reaction energetics. (4) Binding energies of water and hydroxide ions were calculated for mono- and tetracoordinate complexes. Binding energies were very dependent on coordination number. Water was found to bind to [ZII(NH,)~]~+ by 43 kcal/mol (about half of the previously calculated result), and hydroxide binds by 284 kcal/mol. These results have application to zinc enzyme systems such as carbonic anydrase. (5) Proton affinities were also calculated. For the [Zn(NH3)3(OH)]-complexes a value of 167 kcal/mol was calculated with an estimated lower limit of 145 kcal/mol. Acknowledgment. We thank the ONR, N00014-86-K-0557, and the NIH, GM 26462, for financial support. Registry No. ZnH+, 41336-21-4; ZnOH', 22569-48-8; Zn(OH2)2+, 23444-32-8; Zn(NH3)2+, 72155-88-5; (NH3),Zn2+, 103057-99-4; (NH,),Zn(OH)+, 72147-20-7; (NH3),Zn(OH2)2+,721 55-90-9.
Chemical Waves and Light-Induced Spatial Bifurcation in the HgCI2-KI System in Gel Media Ishwar Das,*vt Anal Pushkama, and Namita Rani Agrawal Department of Chemistry, University of Gorakhpur, Gorakhpur-273009, India (Received: December 1 , 1988; In Final Form: May 1 , 1989)
New results are reported on the one-dimensional propagation of a single red/yellow band (ring) of mercuric iodide in gel media in complete darkness. The influence of light on wave propagation has been studied by illuminating the species with light of various wavelengths. A liquid filter containing 12/CC14was used to get monochromatic light (A = 405 nm). A single red band bifurcates into several revert spaced bands when illuminated with natural light having wavelength X < 600 nm. The influence of electrolyte concentrations and temperature on the kinetics of yellow and red wave propagation has been studied that satisfy the relation d2 = kt, where d is the extent of propagation from the initial junction and k and t are the rate constant and time, respectively. The energy of activation for yellow wave propagation is found to be 9.2 kcal/mol. The dependence of bandwidth on time as well as on electrolyte concentrations obeys the relation A d = mt where AMJand m are the width of the yellow band and the slope, respectively. Changes in potential and [H'] during the propagation of the advancing front have been monitored. Results lead to the conclusion that the phenomena involve the transition from one state to another.
Introduction Interdiffusion of one electrolyte into another electrolyte may lead to a rhythmic depoSition,l which is commonly known as the Liesegang phenomena.2 It was noted that a concentration graPresent address: Reader, Academic Staff College, Gorakhpur University, Gorakhpur, India (up to March 1990).
0022-3654/89/2093-7269$01.50/0
dient was necessary for such a type of pattern formation. Experimental evidences are also available for pattern formation in initially homogeneous solutions of electrolytes, in the absence of (1) Hedges, E.S. Liesegang Rings and Other Periodic Structures; Chapman and Hall: London, 1932. (2) Stern, K. H.A Bibliography of Liesegang Rings, 2nd ed.; U S . Government Printing Office: Washington, DC, 1967.
0 1989 American Chemical Society
