J . Phys. Chem. 1985,89, 3185-3188 for C2H4 on an oxygen covered surface is 0.42 for Ru(OO1). Ethylene on oxygen covered Pd( 100) and Pt( 111) has x u values of 0.30 and 0.27, respectively. These x u values are sufficient to classify the adsorption of ethylene on oxygen covered surfaces as x-bonded. The lowest x u values are for the group 11 metals Cu(100) (0.21) and Ag(ll0) + 0 (0.14). Thesevalues imply that there is very little rehybridization and consequently a relatively weak bonding interaction between ethylene and these surfaces. The general trends in the xu parameter are in excellent agreement with the observed reactivities of C2H4 with these metals. Those with high values of n u dehydrogenate ethylene and form strong C2H4-metal bonds, whereas those with low values bind C2H4 weakly and reversibly.'s2 The capacity of the T U parameter to measure CzH4 rehybridization does not depend on the details of the structure of the C2H4 metal complex. For n-bonded C2H4 (nu I 0.4) the degree of rehybridization is small precisely because CzH4 interacts only weakly with the substrate via the n-electrons, regardless of the bond lengths. For strongly interacting C2H4 (nu 1 0.4) vibrational coupling between the external C2H,-metal vibrations (phonons) and the internal v(CC) and 6(CHz) modes should also be small since the external modes are generally less than 500 cm-' and far removed from the 1000-1450-cm-' range of the internal modes. In any case, it it important to remember that CzH4 rehybridization is reflective of the state of bonding of C2H4,and the n u parameter therefore is a measure of the bonding of adsorbed ethylene. The use of the x u parameter to characterize CzH4 adsorption suggests a reassignment of the bands previously assigned for CzH4 on Ru(001) and P d ( l l 1 ) . For Ru(001)," the assignments for di-u-bonded C2H4 were v(CC) = 1330 cm-' and 6(CH2) = 1400 cm-' for a n u value of 0.40. This value is very close to that for n-bonded CZH4 on Ru(001) + 0 of 0.42,"317 yet dehydrogenation on the clean surface implies stronger bonding. We propose the assignment of band I as 1400 cm-' and band I1 as 1110 cm-l on Ru(001) for a n u value of 0.85. Both assignments are tentative, however, since spectra for CzD4 on Ru(001) were not measured.
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It should also be noted that, for CzH4 on both Pd(100) and Ru(001), v(CH) increased on the oxygen covered surface, consistent with less rehybridization of ethylene upon adsorption. For Pd(lll),ls four bandswereobservedforC2H4at 1145, 1229, 1418, and 1502 cm-I. The assignments were v(CC) = 1502 cm-l and 6(CH2) = 1418 cm-' for a x u value of 0.05, which is almost a gas phase value. There were only two bands at 1335 and 953 cm-' for C2D4on Pd( 111) so the assignment of band I and band I1 is unambiguous; the n u value is 0.37, very close to that of Zeise's salt. If the isotope shifts between C2H4 and CzD4 on Pd( 11 1) are equated to those on Pd( 100) (1.58 and 0.93 for 6(CH2) and v(CC), respectively), then the assignments for CzH4 (C2D4) on Pd( 111) should be 6(CH2) = 1502 (953) cm-' and v(CC) = 1229 (1 355) cm-I. This gives a x u parameter of 0.43 for C2H4 on Pd( 111). This xu value implies that C2H4 is n-bonded on Pd( 111) which is unique when compared with di-a-bonding of CzH4 on the other clean transition metals in Table I. The origin of this apparent anomaly is unknown at this time.
Summary The x u parameter is proposed as a measure of the extent of C2H4 rehybridization upon adsorption. This parameter takes into account the vibrational coupling of the v(CC) and 6(CH2) modes and is therefore a more accurate probe of the C-C bond order than the v(CC) frequency alone. Values of the xu parameter range from zero for gaseous C2H4,to 0.38 for Zeise's salt, to unity for C2H4Br2.C2H4 adsorbed on Pt( 11 1) is a good example of di-ubonded CzH4 with its x u value of 0.92, whereas C2H4is n-bonded on oxygen covered, group 8 transition metals with n u values ranging from 0.27 to 0.43. Consideration of the x u parameter may also assist in the assignment of the vibrational frequencies of adsorbed C2H4. Acknowledgment. The authors gratefully acknowledge the support of the National Science Foundation (NSF-CPE 8320072). Registry No. C2H,, 74-85-1.
13C Magic Angle Spinning NMR Study of CO Adsorption on Ru-Exchanged Zeolite Y R. K. Shoemaker and T. M. Apple* Department of Chemistry, University of Nebraska-Lincoln, Lincoln, Nebraska 68588-0304 (Received: March 27, 1985; In Final Form: May 13, 1985)
Three types of adsorbed carbon monoxide are observed on Ru-Y zeolite by I3C magic angle spinning NMR: linear, bridged, and dicarbonyl CO. Samples exposed to CO at room temperature exhibit only linear and dicarbonyl species. At higher adsorption temperature bridged species are formed and a relative increase in dicarbonyl adsorption is observed. A smaller percentage of linear species is produced at higher temperature. The electronic environments of linearly bonded CO are more diverse than those of bridging and dicarbonyl moieties. COzis formed over Ru-Y zeolite upon initial exposure of the catalyst to CO at room temperature, apparently through reaction with unreduced metal oxide.
Introduction The adsorption of carbon monoxide on supported group 820 metals has received considerable attention due to the interest in catalytic hydrogenation of CO. Numerous studies of C O bonding to supported group 8 metals have been conducted using transmission IR,'-7 but few with 13C N M R have been A. C. Yang and C. W. Garland, J. Phys. Chem., 71, 1504 (1957). H. Arai and H. Tominaga, J . Catal., 43, 131 (1976). H. C. Yao and W. G. Rothschild, J . Chem. Phys., 68, 4774 (1978). J. T. Yates, Jr., T. M. Duncan, S. D . Worley, and R. W. Vaughas, J . Chem. Phys., 20, 1219 (1979). (5) J. T. Yates, Jr., T. M. Duncan, and R. W. Vaughan, J . Phys. Chem., (1) (2) (3) (4)
71, 15 (1979). ( 6 ) Y. Tanaka, T. Iizuka, and K. Tanabe, J. Chem. Soc., Faraday Trans. 1, 78, 2215 (1982). (7) B. L. Gustafson, Ph.D. dissertation, Texas A&M University, 1981.
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Several I3C N M R studies have been conducted on metal carbonyls'O and supported metal It is generally accepted that three types of adsorbed CO exist on the surface of a supported-metal catalyst at or near room (8) T. M. Duncan, J. T. Yates, Jr., and R. W. Vaughan, J . Chem. Phys., 71, 3129 (1979). (9) T. M. Duncan, J. T. Yates, Jr., and R. W. Vaughan, J . Chem. Phys., 73, 975 (1980). (10) J. W. Gleeson and R. W. Vaughan, J . Chem. Phys., 78, 5384 (1983). (11) J. B. Nagy, M. van Eenoo, and E. G. Derouane, J . Catal., 58, 230 (1979). (12) E. G. Derouane, J. B. Nagy, and J. C. Vedrine, J . Catal., 46, 434 (1977). (13) W. M. Shirley, B. R. McGarvey, B. Mait, A. Brenner, and A . Cichowlas, J . Mol. Catal., 29, 259 (1985). (14) B. E. Hanson, G. W. Wagner, R. J. Davis, and E. Motell, Inorg. Chem., 23, 1635 (1984).
0 1985 American Chemical Society
3186 The Journal of Physical Chemistry, Vol. 89, No. 15, 1985
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temperature: linear (I), bridged (11), and dicarbonyl (111) specie~.~”q~*~ Several studies’” have indicated that the dicarbonyl 0 I
0
I
I1
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/
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is formed only on isolated atoms, whereas the other species are formed on “crystalline” or bulk metal sites. In pioneering work with I3CN M R to study surface CO, Duncan et al.8*9were able to identify CO species located on isolated atoms and those located on Rh clusters on the basis of spin-lattice relaxation. The isolated dicarbonyl species were found to have shorter Tls than linear and bridged species, to resonate upfield of the linear and bridged forms, and to exhibit considerable motional narrowing. The usefulness of N M R in the study of the adsorption of CO on group 8 metal catalysts derives from the ability to determine the relative amounts of carbon contributing to each N M R resonance line and to identify them by reference to extensive tabulations of N M R chemical shift parameters.15 A difficulty arises, however, when several distinct types of nuclei are present in a solid sample. The anisotropy patterns of the different nuclei overlap and result in a broad featureless spectrum which is difficult to interpret.16 While the conventional N M R spectrum may be useful in many respects, it may not be possible to quantify and unambiguously assign all the resonances. For this reason we have employed the technique of magic angle spinning” to resolve the I3CN M R lines of CO adsorbed on Ru-Y zeolite. The spectra measured in this manner allow one to resolve and quantify all carbonyl species on the catalyst surface. This is crucial to an understanding of product distributions and reaction mechanisms on surfaces. We report here the characteristics of CO adsorption on the surface of Ru-exchanged zeolite Y. Experimental Section
The supported ruthenium catalysts were prepared by ion exchange of Na-Y zeolite (Union Carbide) with 0.03 M RuC1,. The catalysts were filtered, washed, and dried for 12 h in an oven and then calcined in air at 725 K for 2 h. The catalyst was then reduced under flowing hydrogen at 570 K for 24 h. The hydrogen (Linde 99.999%) was passed through molecular sieves and Drierite prior to contacting the catalyst. The resulting catalyst was 8.4 wt % ruthenium. Samples were then outgassed at the reduction temperature to a background pressure of 5 X 10” torr. This treatment was followed by exposure to I3CO(99% MSD isotopes) for 8 h a t a pressure of 700 torr. Samples were subsequently outgassed at room temperature and sealed under vacuum. Static N M R experiments were performed with the catalyst sealed in the N M R sample tube. For magic angle spinning experiments samples were sealed in an A1203rotor under dried nitrogen in a controlled atmosphere chamber. Experiments on test samples confirmed the lack of sample oxidation under magic angle spinning conditions for times greater than 5 days. N o signals were detected in experiments performed with empty rotors. Chemisorption measurements were performed in a glass vacuum system. N M R experiments were performed at 25.1 MHz for 13C on an instrument designed and constructed in-house for solid-state studies. All experiments were performed with phase alternation and spectral subtraction to eliminate coherent noise. For static (nonspinning) spectra a Carr-Purcell sequenceI6 was used to refocus the magnetization after probe recovery was complete. Homogeneous broadening due to 13Cdipolar coupling was not evident in any spectra. Ninety degree pulse widths for carbon were 1.6 ps. Magic angle spinning was performed at speeds from (15) J. B. Stothers, “Carbon-13 NMR Spectroscopy“, Academic Press, New York, 1972. (16) T. M. Duncan and C. Dybowski, Su?J Sci. Rep., 1 , 220 (1981). (17) (a) J. Schaefer and E. 0. Stejkal, J . Am. Chem. SOC.,98, 1031 (1976); (b) I. D. Gay, J . Magn. Reson., 58, 413 (1984).
300
200 100 0 PPM vs TMS
-100 -200
Figure 1. Magic angle spinning spectra of CO on Ru-Y zeolite. (a) CO adsorbed at room temperature and outgassed. 130000 scans at a 1-s repetition period. (b) Temperature ramped to 500 K then cooled under CO and outgassed. 32 500 scans at a 1-s repetition period.
2 to 3 kHz in a cylindrical rotor (Doty Scientific). Between 32 500 and 130000 signal averages were performed at a sampling period of 1 s, which is greater than 5 times T1 for all species.
Resuits and Discussion Magic angle spinning spectra of CO adsorbed on Ru-Y zeolite are shown in Figure 1. The spectrum in Figure l a is of a sample exposed to an initial pressure of 700 torr of CO at room temperature. The catalyst was outgassed at room temperature for 4 h to remove reversibly bound species prior to obtaining the NMR spectrum. Figure 1b shows the N M R spectrum of a Ru-Y zeolite exposed to CO at room temperature and then, while still under a pressure of CO, the temperature was ramped to 500 K and then returned to room temperature. This sample was subsequently outgassed at room temperature. The spectra of Figure 1 show several resonances: a sharp peak at 128 ppm (resonance A) relative to Me,Si; a broad resonance centered at 180 ppm (C); a narrow resonance slightly upfield at 169 ppm (B); and several broad, equally spaced resonances (S), two upfield and two downfield of the component at 180 ppm. These latter peaks are assigned as spinning sidebands of resonance (C) on the basis of variable spinning speed experiments. In the spectrum of CO adsorbed at high temperature, a resonance due to a third species (D) is evident. This activated species is narrow under magic angle spinning and resonates at 203 ppm. Thus in the N M R spectra, we can identify three distinct forms of adsorbates upon room temperature adsorption and four forms upon heating the sample. We assign resonance (A) to gaseous C 0 2 trapped in the zeolite cage. This assignment is based on the following observations: (1) This signal is narrow (13 ppm fwhm) in the static N M R spectrum of the catalyst, while all other resonances are nearly broadened into the base line (Figure 2a). (2) This resonance vanishes with time in samples sealed under vacuum as well as those in the spinning rotor, indicative of a species diffusing out of the zeolite cages. (3) The chemical shift is 128 ppm in agreement with the value for gaseous CO2.l5 It is important to note that, while this signal vanishes on the time scale of several days, all other 13C signals are time inde-
The Journal of Physical Chemistry, Vol. 89, No. 15, 1985 3187
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TABLE I: Summary of NMR Data for Adsorbed Specieso line width relative %' species shiftb (fwhm) 298 K 500 K linear (~b =180 40 92 74 ull = -89 ~ 2 = 2 310 ~ 3 = 3 320 dicarbonyl uim = 203 7 8 18 bridged uim = 168 10 8
181
"Shifts are in ppm vs. Me& (positive = downfield). buncertainties are f5 ppm for isotropic shifts and f2O ppm for anisotropy components. Corrections for bulk susceptibility have not been made. CRelativepercent of irreversibly adsorbed CO f 5%.
300 200 100
0
-100 -200
PPM vs T M S Figure 2. Static Carr-Purcell spectra of CO over Ru-Y zeolite. (a) Exposed to 700 torr of CO at room temperature and then outgassed. (b) With an overpressure of 700 torr of CO.
pendent. Therefore, this C 0 2is not formed by oxidation of existing carbon species on the surface. We do not believe that the C 0 2 derives from CO disproportionation with resultant surface carbide formation. Surface carbide species have been reported to have chemical shifts ranging from 200 to 400 ppm and line widths of 100 ppm.'* The line broadening due to chemical shift anisotropy would be removed by magic angle spinning, leaving the distribution of isotropic shifts as the major broadening mechanism. By comparison with other surface bound species, we do not expect an isotropic chemical shift dispersion much larger than 30-40 ppm. A carbide species with this line width and an intensity at least as large as that of the C 0 2resonance should be detectable. Since an N M R resonance in this range is not observed, we discount CO disproportionation as the source of C 0 2 . Rather, it would appear that an oxidizing species is present in the catalyst surface initially upon exposure to CO. A sample of unexchanged zeolite was prepared in the identical manner as the Ru exchanged sample in Figure 2a. This sample showed no I3CN M R resonance of any kind. Thus the presence of metal is necessary for this oxidation to Cot and, indeed, for any irreversible adsorption to occur. We believe, therefore, that a t least some of the Ru present initially upon exposure to CO is in the form of a ruthenium oxide. It is interesting that a surface species is reduced by CO at room temperature, but not reduced by hydrothermal processes during reduction. A rough estimate of a lower limit on the number of these species originally present on the catalyst surface can be made by counting the spins contributing to the C 0 2 resonance. We obtain a value of 3.3 X lOI9 spins/g which corresponds to 7% of all ruthenium atoms in the catalyst. This should be taken as a lower limit since we are discounting the C 0 2which has diffused out of the zeolite cages during data acquisition. The remaining carbon resonances have isotropic values in the range expected for carbonyls. We can discount the assignment of any of these resonances to gaseous CO. We observe a gaseous CO resonance at 181 ppm, provided an overpressure of CO is maintained; however, this resonance is always narrow even in nonspinning experiments (Figure 2b). Outgassing of this sample a t room temperature for less than 1 h results in quantitative
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(18)T.M.Duncan, P. Winslow, and A. T.Bell,Chem. Phys. Lett., 102, 163 (1983).
removal of the gaseous CO peak. The resonance centered at 169 ppm (B in Figure 1) is assigned to dicarbonyl species on Ru based upon the following arguments: (1) In the magic angle spinning experiment, resonance B is completely narrowed as evidenced by the lack of spinning side bands. This indicates that the chemical shift anisotropy for this species is less than or on the order of the 2.6-kHz spinning speed. The dicarbonyl species observed by static experiments on Rh/ alumina exhibited a line width of 1.85 kHzS9 (2) Saturation-recovery experiments were performed on both samples. Species B exhibited a much shorter T I than the other species. The work of Duncan et ale9showed clearly that, due to their motional properties, dicarbonyl species on surfaces tend to have shorter spin-lattice relaxation times than those of carbonyls on metal rafts (linear and bridged). (3) On Rhalumina the narrowed dicarbonyl resonates 22-ppm upfield of the species associated with metal rafts (linear and bridged).9 Our results are in agreement with this shift. We assign the broad resonance centered at 180 ppm (C in Figure 1) and its associated spinning side bands (S) to linear CO on Ru. The anisotropy is 409 ppm, calculated from the spinning side band intensities by the method of Herzfeld and Berger.19 This anisotropy and the isotropic chemical shift are on the order of those expected for linear CO, as observed for standard metal carbonyls in the solid state. One aspect of the signal of the linear species warrants further comment. The line width of this species is considerably larger than that of the other carbonaceous adsorbates. Under magic angle spinning conditions this line width is attributable to a distribution of isotropic chemical shifts. Linearly bonded CO is, therefore, experiencing a greater variety of electronic environments over the entire sample than are other CO species. One might expect the electronic structure to change as the position of the CO moves from the center to the edges of a metal particle. The environment of lone dicarbonyls on single metal atoms would not show such heterogeneity. We assign the resonance of the activated species (D in Figure l b ) to bridged carbonyls on the following basis: (1) The isotropic shift of this species is downfield of the linear CO species by 23 ppm. The work of Gleeson et al.1° on group 8 metal carbonyls showed that bridged carbonyls have isotropic shifts downfield of linear species, due to the strong contribution of the paramagnetic shielding term to u,,. (2) The resonance is narrowed by magic angle spinning at 3 kHz. Increased shielding along the unique axis results in drastically reduced shift anisotropies for bridged carbonyls, which are typically about 140 ppm compared to 400 ppm for linear C0.lo At these spinning speeds, one would expect to narrow a bridged species to its isotropic value, since its anisotropy is on the order of the spinning speed. (19) J. Herzfeld and A. E. Berger, J . Chem. Phys., 73, 6021 (1980). (20)In this paper the periodic group notation is in accord with recent actions by IUPAC and ACS nomenclature committees. A and B notation is eliminated because of wide confusion. Groups IA and IIA become groups 1 and 2. The d-transition elements comprise groups 3 through 12, and the p-block elements comprise groups 13 through 18. (Note that the former Roman number designation is preserved in the last digit of the new numbering: e.g., 111 3 and 13.)
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J . Phys. Chem. 1985,89, 3188-3189
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A summary of our N M R results is presented in Table I. Integration of the line shapes of the sample exposed at room temperature yields a total spin density of 1.3 X lozoCO’s per gram of catalyst. Using a CO/Ru ratio of 1 for linear CO and a CO/Ru ratio of 2 for the dicarbonyl, we obtain a dispersion of 30%. Chemisorption measurements yielded a dispersion of 27%. Upon high-temperature exposure, a greater number of dicarbonyls are formed, indicating that dicarbonyl adsorption is activated. A bridged species is also formed, whereas the relative proportion of linear species decreases. We cannot determine whether the bridged species derives directly from the linear species, or whether the adsorption of the bridged species precludes the presence of a linear species a t a particular site. Since no change is evident
in the metal surface area after heating, this change in C O distribution cannot be attributed to a change in metal particle size. We have shown that I3C magic angle spinning N M R can be used to identify and quantify CO adsorbates. This technique holds promise for studying differential reactivity of adsorbed species. In addition, since dicarbonyl species may only bond to isolated atoms, the degree of atomic dispersion may be detected by 13C NMR.
Acknowledgment. This work was supported, in part, by a Cottrell Grant of the Research Corporation and by a Grant-in-Aid from the University of Nebraska Research Council. Registry No. CO, 630-08-0; Ru, 7440-18-8.
Critical Behavior of Hydrogen Bonds Observed in High-pressure Nuclear Quadrupole Resonance Studies J. Stankowski,* M. Maekowiak, P. Koziol, and J. Jadiyn Institute of Molecular Physics, Polish Academy of Sciences, Smoluchowskiego 17, 60-1 79 Poznait, Poland (Received: December 3, 1984; In Final Form: May 3, 1985) The high-pressure effect on NQR frequencies of 35Clnuclei has been studied for the complexes of pentachlorophenol with nitrogen bases at 77 K. The correlation between the value of pressure frequency coefficient and the degree of proton transfer has been found. In the vicinity of the critical point of the hydrogen bond (complexes with 50% proton transfer) the anomaly of the pressure frequency coefficient has been observed. Introduction
Changes in the electron density distribution in both the donor and acceptor molecules are one of the consequences of hydrogen-bond formation. The NQR spectroscopy, being particularly sensitive to subtle changes in electron density, is a valuable tool in studies of hydrogen bonds in the solids. A proper selection of proton donor and acceptor enables one to modify the position of proton inside the bridge which reveals itself as a change in the NQR frequency. This effect had been reported first by Chihara and Nakamura.’ Studies by numerous author^^-^ showed a direct relation between the N Q R frequency and the degree of proton transfer in the hydrogen bonds. The aim of the present paper was to correlate the high-pressure effect on the N Q R frequency with the degree of proton transfer in hydrogen bonds. The N Q R measurements were carried out for 35C1nuclei in pentachlorophenol (PCP) complexes with nitrogen bases. The possibility of changing the acceptor properties within a wide range of pKa makes these complexes model hydrogen-bonded systems. Thus, both weak hydrogen bonds of the covalent type A-H--B as well as strong ionic bonds with transferred protons A---H-B+ can be studied within one class of compounds. The intermediate range, in which there is the 50% proton transfer, seems to be particularly interesting since in this range one can expect some critical phenomena. Results and Discussion
Complexes of PCP with nitrogen bases were synthesized by the method described by Grech et aL3 The N Q R measurements were made with a pulse ISSh-1-12 spectrometer. The positions of resonance lines were determined with the accuracy of f5 kHz. The pressure system, enabling one to apply pressure up to 500 MPa, was described in a previous (1) H. Chihara and N. Nakamura, Bull. Chem. SOC.Jpn., 44, 1980 (1971). (2) J. Kalenik, Ph.D. Thesis, Wrwlaw University, 1982. (3) E. Grech, J. Kalenik, and L. Sobczyk, J. Chem. SOC.,Faraday Tram. I.., 75. -, 1587 ... (19191. -, (4) J. Pietrzak, B. Nogaj, Z. Dega-Szafran, and M. Szafran, ACIUPhys. Pol., A, 52, 779 (1977). (5) E. Grech, J. Kalenik, Z. Malarski, and L. Sobczyk, J . Chem. SOC., Faraday Trans. 1, 79, 2005 (1983). \ - -
paper.6 NQR spectra of most investigated complexes had been registered first by J. Kalenik at normal pressure;2 frequencies determined by us (Table I) agree with his results. Spectra of investigated complexes consist of five lines corresponding to the five chlorine atoms of the phenolic ring. To eliminate the “crystal effect”, one usually introduces an average value of the NQR frequency (B~). Since the electron density at five chlorine atoms is variously modified by the transformation taking place inside the hydrogen bond, the interpretation of an average value of the resonance frequencies may bring about some doubts. This may be especially significant in the case of pressure studies in which the changes in the electric field gradient caused by high pressure are highest in the vicinity of the hydrogen bond. The dependence of the pressure coefficient of 35ClNQR frequency (E)v1/ap)T=77determined for the lowest resonance frequency on pKa of base is shown in Figure 1. The dependence of the NQR frequency of the same resonance line plotted vs. basicity of nitrogen bases is shown in Figure 2. The pressure coefficient of the NQR frequency (Table I) is negative for weak hydrogen bonds and changes insignificantly with the increasing pKa. At a pKa of about six the critical increase of the absolute value of ay/dp by 1 order of magnitude is observed. It reaches an extreme value for complexes in which the degree of proton transfer amounts to about 50%. Further increase of pK, gives an increase of the pressure coefficient from large negative to positive values. For complexes of A-m-H-B’ type the pressure coefficient has a positive value. The electric field gradient at the investigated chlorine nucleous is a function of proton position in the hydrogen bridge. The electric field created by the hydrogen-bond dipole causes a change in polarization of the C-Cl bond, thereby shifting the N Q R frequency. That is why in the interpretation of the high-pressure effect we shall neglect, in the first approximation, all intermolecular interactions (crystal effect) and we shall discuss only the pressure effect on the dipole moment of the hydrogen bond. The high-pressure studies of ferroelectric crystals showed that the hydrogen bond itself can be relatively easy deformed and its compressibility coefficient amounts to aRo...o/ap = 0.003 nm. (6) M. MaEkowiak, J. Stankowski, and M. Zdanowska, J . Magn. Reson., 31, 109 (1978).
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