11420
J. Phys. Chem. 1996, 100, 11420-11426
ESCA and Solid-State NMR Studies of Ionic Complexes of 1,8-Bis(dimethylamino)naphthalene Krzysztof Wozniak,†,‡ Heyong He,‡ Jacek Klinowski,*,‡ Tery L. Barr,§ and Piotr Milart| Department of Chemistry, UniVersity of Warsaw, ul. Pasteura 1, 02-093 Warszawa, Poland, Department of Chemistry, UniVersity of Cambridge, Lensfield Road, Cambridge CB2 1EW, U.K., Department of Materials, Laboratory for Surface Studies, UniVersity of Wisconsin-Milwaukee, Milwaukee, Wisconsin 53201, and Department of Chemistry, Jagiellonian UniVersity, ul. Ingardena 3, 30-060 Krakow, Poland ReceiVed: December 14, 1995; In Final Form: April 25, 1996X
A series of ionic complexes of 1,8-bis(dimethylamino)naphthalene (DMAN) with organic and inorganic acids such as HF, HCl, HBr, HI, HClO4, HBF4, HNCS, CCl3COOH, 4-nitrosophenol, 4-(dicyanomethyl)nitrobenzene, and 1,2-dichloromaleic acid have been studied by electron spectroscopy (ESCA) and 13C and 1H MAS NMR. Structural and spectroscopic features of this series of compounds are discussed and correlated. We demonstrate the influence of strong ionic [N-H‚‚‚N]+ hydrogen bonds in proton sponges on the binding energies of core electrons of donor and acceptor atoms in the ionic complexes and give the detailed X-ray structure of the complex of DMAN with 4-(dicyanomethyl)nitrobenzene.
Introduction Electron spectroscopy for chemical analysis (ESCA) is a powerful technique for the study of the near-surface region of solids (down to the depth of ca. 50 Å).1-3 When a specimen is irradiated, under ultrahigh-vacuum conditions, by soft monochromatic X-rays, photoelectrons with sufficient kinetic energy escape from the sample and their energy spectrum is measured. The binding energy of each core electron is characteristic of the atom to which it was bound. ESCA spectra are very informative because the binding energies are affected by the chemical environment of the respective atoms. In different chemical environments the binding energies exhibit a “chemical shift” due to the different contributions of the charge density of the valence electrons at the core orbitals. The present work follows two earlier papers4,5 in demonstrating to the general chemical audience the value of ESCA in the study of systems containing the hydrogen bond. We show that ESCA provides directly the parameters characterizing the donor and acceptor atoms participating in the bond and leads to a set of parameters with which the strength and symmetry of hydrogen bonds can be defined. Such parameters have not so far been generally agreed upon by the workers in the field. As far as ESCA experts are concerned, these three papers bring attention to new materials and problems to which the technique may be applied. Reference 4 demonstrated the general potential of ESCA using a complex of 1,8-bis(dimethylamino)naphthalene (DMAN), the parent molecule of a class of compounds known as proton sponges, with dichloromaleic acid. Reference 5 dealt with a wide series of 1,8-bis-substituted derivatives of naphthalene and structural correlations brought about by the change of ligands at positions 1 and 8. The present work addresses a much more subtle effect: the influence of strong cationic [N-H‚‚‚N]+ hydrogen bonds on the binding energies of core electrons of the donor and acceptor atoms in ionic complexes of DMAN with organic and inorganic acids. We feel that it is remarkable †
University of Warsaw. University of Cambridge. § University of Wisconsin-Milwaukee. | Jagiellonian University. X Abstract published in AdVance ACS Abstracts, June 15, 1996. ‡
S0022-3654(95)03732-4 CCC: $12.00
that, despite the smallness of the effect (given the screening of the [N-H‚‚‚N]+ proton by the bulky N,N-dimethylamine groups), solid-state NMR and ESCA can detect any correlations caused by the change of the counterion. Having demonstrated a correlation between 13C CP/MAS (CP ) cross-polarization, MAS ) magic-angle spinning) NMR chemical shifts in a series of 1,8-bis-substituted derivatives of naphthalene,5 we now consider a narrower series of ionic complexes of DMAN in order to look for possible regularities in variation of binding energies, 1H and 13C MAS NMR chemical shifts, and structural parameters. We have examined complexes of the DMAN cation (DMANH+) with the following anions: (a) F- (DMANH+F-), (b) Cl- (DMANH+Cl-), (c) Br(DMANH+Br-), (d) I- (DMANH+I-), (e) ClO4- (DMANH+ClO4-), (f) CCl3COO- (DMANH+CCl3COO-), (g) 4-nitrosophenol (DMANH+pNPh-), and (h) 4-(dicyanomethyl)nitrobenzene (DMANH+pCNPhN-). Literature data for the complexes of DMAN with tetrafluoroborate acid HBF4 (DMANH+BF4-),5 1,2-dichloromaleic acid (DMANH+ClMH-),4 and HNCS (DMANH+NCS-)5 will be used in addition. DMAN complexes with HF, HCl, and HBr are hydrates with two H2O molecules per one molecule of the complex. Chemical formulas of the complexes studied are shown in Figure 1. Proton sponges are model compounds which have attracted considerable interest due to their very high proton affinity.6,7 With mineral or organic acids they form very stable ionic complexes containing intramolecular [N‚‚‚H‚‚‚N]+ hydrogen bonds. Properties of these hydrogen bonds are used in discussion of the role of hydrogen bonding in enzymatic catalysis.8,9 There are at present more than 100 published papers on intramolecularly hydrogen-bonded proton sponges, and the interest in them continues. Experimental Section Solid-State NMR. 1H MAS, 13C MAS, and 1H-13C CP/ MAS spectra were recorded at room temperature and 123 K at 399.9 MHz for 1H and 100.6 MHz for 13C, using a Chemagnetics CMX-400 spectrometer. Zirconia rotors were spun in nitrogen gas at different rates in order to identify sidebands and resonance overlaps. 1H spectra were measured with spinning at 9-12 kHz using 2 µs (30°) pulses and 3 s recycle delays. © 1996 American Chemical Society
Ionic Complexes of 1,8-Bis(dimethylamino)naphthalene
J. Phys. Chem., Vol. 100, No. 27, 1996 11421 TABLE 1: Crystal Data and Structure Refinement Details identification code
Figure 1. Numbering scheme for the series of ionic complexes of DMANH+ with (a) F- (DMANH+F-), (b) Cl- (DMANH+Cl-), (c) Br(DMANH+Br-), (d) I- (DMANH+I-), (e) ClO4- (DMANH+ClO4-), (f) CCl3COO- (DMANH+CCl3COO-), (g) 4-nitrosophenol (DMANH+pNPh-), and (h) 4-(dicyanomethyl)nitrobenzene (DMANH+pCNPhN-).
Single contact 1H-13C CP/MAS experiments were performed with 4 ms contact times and short-contact-time experiments with 50 µs contact times. The 1H π/2 pulses were typically 2.5 µs, the recycle delay was 3 s, and the MAS rate was 5-9 kHz. Dipolar-dephased 1H-13C spectra were recorded with a 50 µs delay prior to acquisition, 4 ms contact time, and 3 s recycle delays. Conventional MAS spectra contain resonances from all carbons, dipolar-dephased spectra identify resonances from quaternary carbons, and short-contact-time CP spectra reveal protonated carbons. ESCA. ESCA spectra were recorded on a Hewlett-Packard (HP) 5950A spectrometer with a high-resolution X-ray monochromator. A conventional Al KR anode was used as a source of X-ray radiation. The background pressure during analysis was ca. 1 × 10-9 Torr. The materials under study are insulators, thus producing distinctive charging shifts.10 These were removed and the binding scale was established using a combination of electron flood guns and fixing the C(1s) binding energy of the hydrocarbon part carbons at 284.6 eV. This procedure has been successfully applied in a number of previous studies.2,11 Broad survey scans with the scan range from 0 to 1000 eV were run to identify all elements, and then detailed scans (20 eV wide) were recorded and deconvoluted to establish precise peak location. The C(1s) line used for the charge referencing was measured at the beginning, in the middle, and at the end of the data collection, and its position was constant with time. X-ray Diffraction. Crystals of DMANH+pCNPhN- suitable for X-ray work were crystallized from acetonitrile by slow evaporation. X-ray measurements were made on an EnrafNonius CAD 4 diffractometer using monochromated Mo KR radiation and the ω-2θ scan mode. Three standard reflections were monitored per every 100 reflections collected and showed no significant decrease in their intensities during the data collection time. The data were corrected for the Lorentz and polarization effects, and the structure was solved by direct methods12 and refined using SHELXL-93.13 The refinement was based on F2 for all reflections except those with very negative F2. Weighted R factors ωR and all goodnesses-of-fit S values are based on F2. Conventional R factors are based on F with F set to zero for negative F2. The criterion Fo2 > 2σ(Fo2) is used only for calculating R factors and is not relevant to the choice of reflections for refinement. R factors based on
DMANH+pCNPhN-
empirical formula formula weight color temperature [K] wavelength [Å] crystal system space group
C23H23N5O2 401.46 red 293(2) 0.71073 triclinic P1h
a [Å] b [Å] c [Å] R [deg] β [deg] γ [deg] volume [Å3] Z density (calc) [Mg/m3] absorption coefficient [mm-1] F(000) crystal size [mm3] θ range for data collection [deg] index ranges
9.503(1) 9.646(1) 11.858(1) 73.88(1) 82.57(1) 84.84(1) 1033.8(2) 2 1.290 0.085 424 0.20 × 0.25 × 0.30 1.8 to 21 -9 e h e 9
reflections collected independent reflections R (int) refinement method data/restraints/parameters goodness-of-fit final R indices [I > 2σ(I)] R1, ωR2 R indices (all data) R1, ωR2 largest diff. peak/hole [e Å-3]
-9 e k e 1 -11 e l e 11 2620 2126 0.05 full-matrix least-squares on F2 2107/0/363 1.030 0.0558, 0.140 0.0847, 0.1687 0.225/-0.232
F2 are about twice as large as those based on F, and R factors based on all data are even larger. Scattering factors and absorption coefficients were taken from Tables 6.1.1.4 and 4.2.4.2 in ref 14. Experimental details of data collection and refinement are summarized in Table 1. Results and Discussion Solid-State NMR. Except for DMANH+Br- and DMANH+I-, the 1H MAS NMR spectra of the complexes of DMANH+ (Figure 2) contain either separate peaks or shoulders from hydrogens attached to the aliphatic (lower chemical shifts in the range 1.8-4.9 ppm) or aromatic (3.5-8.3 ppm) carbons. The most striking feature is the substantial 1H chemical shift of a small peak from the acidic proton (16.9-20.7 ppm), the largest 1H chemical shift ever recorded. The 13C resonances have been assigned by the analysis of the dipolar-dephased and short-contact-time spectra as well as by reference to the literature.15-18 The final peak assignment in the MAS NMR spectra for all the compounds in the series is shown in Table 2. The average carbon chemical shifts in ionic complexes of DMAN are given in Figure 3a. The values given in parentheses are standard errors of the average values, while the values in brackets are ranges of variation for a given parameter. As a result of the isolation of the DMANH+ cation, the variation of all the parameters is quite small. The average chemical shift of the carbon nuclei in the peri positions (1 and 8) is 145.1(5) ppm, and the range of variation of this line is ca. 5.2 ppm. Similarly, the average chemical shifts of C(9) and C(10) are 118.7(4) and 134.9(2) ppm with the ranges of variation of 2.9 and 2.2 ppm, respectively. The protonated carbons C(2), C(3), and C(4) give lines at 122.3(5), 127.1(3), and 129.1(4) ppm with the ranges of variation 4.7, 2.7, and 3.8 ppm. The
11422 J. Phys. Chem., Vol. 100, No. 27, 1996
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TABLE 2: Final Peak Assignment in the MAS NMR Spectra of the Ionic Complexes of DMANa C(1), C(8)
C(2), C(7)
C(3), C(6)
C(4), C(5)
C(9)
C(10)
CMe
Hac
Har
Hal
F-
anion
144.6
120.6
126.3
128.5
117.5
135.7
18.1
7.5
3.8
ClBr-
145.9 146.0
124.8 125.2
128.8 128.3
128.8 128.3
119.5 120.0
133.9 133.7
47.7 45.7 46.5 46.6
19.5 19.0
7.7 8.3
IClO4-
143.4 143.7
122.2 120.5
127.0 127.1
128.9 128.3
117.4 117.7
135.0 135.4
20.7 18.8
4.3 3.5
CCl3COOpNPhpCNPhN-
146.9 145.6 143.9
122.6 123.0 120.3 122.2
126.7 125.1 127.6
130.4 130.0 131.9
119.8 120.1 118.1
134.1 135.7 135.9
47.2 46.6 43.6 46.4 46.5 48.5 46.6
4.9 4.9 2.7 4.3 3.5
16.9 18.6 18.6
6.9 6.2 7.2
2.5 3.3 1.8
a
For data on DMANH+BF4-, DMANH+ClMH-, DMANH+NCS-, and DMAN see refs 4, 5, 17, 15, and 18.
a
b
Figure 3. (a) Average 13C CP/MAS NMR chemical shifts in the ionic complexes of DMAN. Standard errors are given in parentheses and the ranges of variation, calculated from the data in Table 2 and refs 4, 5, 17, 15, and 18, in square brackets. (b) Relationship between chemical shift of the acidic protons and an average chemical shift of the C(1) and C(8) carbons.
Figure 2. 1H MAS NMR spectra of ionic complexes of DMANH+ with (a) F- (DMANH+F-), (b) Cl- (DMANH+Cl-), (c) ClO4(DMANH+ClO4-), (d) CCl3COO- (DMANH+CCl3COO-), (e) 4-nitrosophenol (DMANH+pNPh-), and (f) 4-(dicyanomethyl)nitrobenzene (DMANH+pCNPhN-).
chemical shifts of carbons in the methyl groups are 46.9(2) and 45.8(4) ppm with the variation ranges of 2.4 and 3.6 ppm. The variation of the chemical shift decreases in the following sequence: peri positions (1 and 8) > positions 2 and 7 > positions 4 and 5. The variations of chemical shifts are a measure of the sensitivity of a given position to small changes in the properties of the [N-H‚‚‚N]+ hydrogen bond. The increased range of variation of carbon chemical shifts in positions 4 and 5 confirms that substantial polarization of the
whole moiety occurs under protonation and the formation of the [N-H‚‚‚N]+ hydrogen bond. A further confirmation is provided by the relationship between the chemical shifts of the acidic protons and the average chemical shift of carbon nuclei in positions 1 and 8 (Figure 3b). Increased acidity of the proton is associated with decreased chemical shift of the peri carbon nuclei. The reason may be that in the reference DMAN molecule there is a larger degree of conjugation between the lone electron pairs of the nitrogen atoms and flexible π-electron density of the aromatic rings. This conjugation seems to be reduced upon protonation. The effect is transmitted over the whole moiety, which is well illustrated in a series of relations defined in Figure 4. There are particular reasons why all these relationships are not stronger. First of all, the nature of this phenomenon is such that a given small perturbation such as changes in interactions of the [N-H‚‚‚N]+ hydrogen bond with a counteranion causes a relaxation of this effect over the whole
Ionic Complexes of 1,8-Bis(dimethylamino)naphthalene
J. Phys. Chem., Vol. 100, No. 27, 1996 11423
Figure 4. Relationships between the average chemical shift of the C(1) and C(8) carbons and (a) average chemical shift of the C(2) and C(7) carbons, (b) chemical shift of C(9), and (c) chemical shift of aromatic hydrogen atoms.
Figure 5. Differences between chemical shifts in DMAN and the average values of chemical shifts of equivalent carbon atoms in DMANH+.
cation. As a result of the complex structural reaction of the cation, it is difficult to isolate the one or two most sensitive parameters. Another reason for the weakness of the trends in NMR spectral parameters is the rather low precision of the technique. Improved precision would lead to clearer relations between the various chemical shifts. The extent of the changes in the DMAN moiety upon protonation may be measured by comparing the chemical shifts in the series of complexes of DMAN with the values for the reference DMAN base (Figure 5). The formation of the [N-H‚‚‚N]+ hydrogen bond in the ionic complexes of DMAN causes a redistribution of electron density over the whole moiety. In the highly polarizable DMANH+ cation the electron density increases in its central part and decreases on the periphery. This is why all of the chemical shifts of protonated carbons are much higher in the complexes than in DMAN itself and why all of the chemical shifts from quaternary nuclei are higher in the DMAN base than in its cations. This may be correlated with the charge density and the Laplacian of charge densities.18,19 In terms of the nuclear shielding constants or chemical shifts, the largest effect of protonation and formation of an intramolecular [N-H‚‚‚N]+ hydrogen bond is found in positions 2 and 7, 4 and 6, and 1 and 8 of the naphthalene rings. Upon protonation the entire fragment acquires a positive electric charge, which undergoes partial delocalization through polarization of flexible π-electron density of the aromatic fragment. ESCA. The N(1s) ESCA spectra of the ionic complexes of DMAN in Figure 6 are plots of electron binding energy versus the number of electrons in a small, fixed, energy interval. The substantial widths (3-4 eV) of all the peaks indicate a superposition of singular peaks. All ionic complexes of DMAN contain two nitrogen atoms with the acidic proton, normally
Figure 6. N(1s) ESCA spectra of ionic complexes of DMANH+ with (a) F- (DMANH+F-), (b) Cl- (DMANH+Cl-), (c) Br- (DMANH+Br-), (d) I- (DMANH+I-), (e) ClO4- (DMANH+ClO4-), (f) CCl3COO(DMANH+CCl3COO-), (g) 4-nitrosophenol (DMANH+pNPh-), and (h) 4-(dicyanomethyl)nitrobenzene (DMANH+pCNPhN-).
asymmetrically located in the [N-H‚‚‚N]+ hydrogen bridge. The nitrogens give well-resolved lines at average positions of 401.6(2) and 399.9(2) eV for the protonated and nonprotonated nitrogen atoms. The average difference between these lines is 1.7(1) eV and measures an average difference between the average donor and acceptor nitrogen atoms in the complex. This difference may be treated as the asymmetry parameter of the hydrogen bond. The value of the nitrogen binding energy in some pure 1,8-bis(dimethylamino)naphthalenes such as 4-nitroDMAN and 4-picrylDMAN is 399.3 eV5 and can be used as a reference binding energy of nitrogen in pure bases (without
11424 J. Phys. Chem., Vol. 100, No. 27, 1996
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Figure 7. Relationships between (a) binding energies of the nitrogen donor and acceptor atoms and (b) binding energies of the donor nitrogen atoms and the 1H NMR chemical shifts of acidic protons.
the hydrogen bond). The difference between the binding energies of nitrogen atoms in the complexes with the intramolecular [N-H‚‚‚N]+ hydrogen bonds and the 399.3 eV reference may therefore be used to measure the strengths of the influence of hydrogen bonds on the donor and acceptor core electrons. In the case of the ionic complexes of proton sponges these differences are on average 2.3 and 0.6 eV for the donor and acceptor atoms, respectively. There is a weak relationship between the binding energies of the nitrogen donor and acceptor atoms (Figure 7). An increase of one of these binding energies is followed by an increase of the other. There is also a correlation between the binding energies of the donor nitrogen atoms and the 1H MAS NMR chemical shifts of the acidic protons (Figure 7). In all of these cases the surface-oriented nature of ESCA is also causing the detection of environmentally induced effects due to chemisorbed oxygen and water compounds. These molecules have been shown to interact with carbonaceous systems to create surface-oriented components with C-O-C, C-O-H, and CdO bond environments. Work on the determination of the environmental effect of oxygens in organic compounds is in progress. The shapes of the N(1s) peaks in the complexes of DMANH+ with pNPh- and pCNPhN- are rather complicated. The first line shape results from the fact that the pNPh- moiety contains an additional nitrogen atom in the -NO group. In fact, this is a 1:2 complex of DMAN, and as a consequence the pnitrosophenol moiety coexists in the crystal lattice with the pNPh- anion in the form of the neutral molecule. The nitrogen atom from the anion gives a peak at 399.5 eV, while the peak at 402.5 eV comes from the neutral pNPhH moiety. The complex with pCNPhN- contains three additional nitrogen atoms: two in the cyano groups and one in the nitro group. The peak from the nitro group is at 406.0 eV, while the nitrogens from the cyano groups appear at 398.9 and 398.1 eV. The most important features of the O(1s) ESCA spectra of the ionic complexes of DMAN (not shown) are as follows. Firstly, the line widths suggest their composite nature (with the possible exception of DMANH+ClO4-, where the narrow peak may result from dynamic disorder of the anion or very high point symmetry of the anion site). Secondly, the complexes of DMAN with F-, Cl-, and Br- contain two distinguishable oxygen atoms belonging to two independent water molecules. Thirdly, the peaks from two different oxygen atoms in CCl3COOcan easily be distinguished similarly as the peaks from oxygens in pNPh- and pCNPhN-. We note that DMANH+I-, which
Figure 8. (a) Labeling of atoms and ORTEP illustration of thermal motions of atoms in DMANH+pCNPhN-; (b) 3D arrangement of molecules showing a projection of the unit cell along the X axis.
does not contain water of crystallization, also gives an O(1s) spectrum. This is because ESCA is sufficiently sensitive to monitor traces of oxygen and water vapor adsorbed on the surface of the crystal.
Ionic Complexes of 1,8-Bis(dimethylamino)naphthalene
J. Phys. Chem., Vol. 100, No. 27, 1996 11425
Figure 9. Relations between (a) chemical shift of acidic proton [ppm] and the donor hydrogen N-H bond length [Å]; (b) chemical shift of acidic proton [ppm] and the hydrogen acceptor H‚‚‚N distance [Å]; (c) N-H‚‚‚N angle [°] and an average chemical shift of the C(3) and C(6) atoms; (d) the difference (delta) between ESCA binding energies of the donor N(1) and acceptor N(2) atoms and average 13C NMR chemical shift of the methyl carbons. 1 stands for the DMANH+ complex with BF4-; 2 with Br-; 3 with hydrogen 1,2-dichloromaleate anion; 4 with SCN-; and 5 with pCNPhN-.
X-ray Diffraction. The complex of DMAN with 4-(dicyanomethyl)nitrobenzene (DMANH+pCNPhN-) crystallizes in the triclinic P1h space group with two molecules in the unit cell (Table 1). The unit cell parameters are a ) 9.503(1) Å, b ) 9.646(1) Å, c ) 11.858(1) Å, R ) 73.88(1)°, β ) 82.57(1)°, and γ ) 84.84(1)°. All positions of hydrogen atoms were obtained from difference electron density maps. The labeling scheme of atoms in the asymmetric unit and an ORTEP illustration of thermal motions of atoms are shown in Figure 8a. The coordinates of atoms and the equivalent/isotropic temperature factors, structural parameters for DMANH+pCNPhN-, and anisotropic thermal parameters as well as the observed and calculated structural factors have been deposited as supporting information. The asymmetric unit of DMANH+pCNPhN- consists of DMANH+ cations that are almost perpendicular to the pCNPhNanion. The angle between the best planes for the aromatic fragments in both moieties is 82.30(7)°. The crystal structure of DMANH+pCNPhN- is built up from pairs of stacked cations and anions located more or less at right angles to one another. In each pair, cations and anions are placed in an antiparallel manner. The 3D arrangement of molecules in the crystal lattice is shown in Figure 8b. As a result of strong electrostatic interactions, some relatively short intermolecular contacts occur. The shortest are between the O(2a) oxygen from the anion and the H(132) and H(123) hydrogens from the cation. They can be considered as weak C-H‚‚‚O hydrogen bonds donated by the cation. The DMANH+ cation is almost planar [with the largest deviation from planarity being -0.023(3) Å for N(2) and 0.0022(2) Å for C(2)] and almost symmetric with all the structural parameters in the left and right parts of the moiety deviating less than 3σ from one another. Also the anion is
symmetric with its local 2-fold axis passing through the C(7), C(1), C(4), and N(3) atoms. The hydrogen bridge in the cation is slightly asymmetric [N(2)H(1n) ) 1.26 Å and N(1)‚‚‚H(1n) ) 1.37(4) Å], but the difference between the two N‚‚‚H contacts is less than 3σ. The N(2)-H(1n)‚‚‚N(1) angle and N(1)‚‚‚N(2) contacts are 156(3)° and 2.573(3) Å, respectively. On the other hand, ESCA suggests the presence of two different amine nitrogens. The small asymmetry of the hydrogen bond is not transmitted over the rest of the cation. The values of structural parameters for the cation and anion are similar to those in other DMAN complexes.20-35 We shall only mention that as a result of protonation and hydrogen bond formation, the entire cation has become more planar than the DMAN molecule. Other significant structural changes in the cation are (i) the lengthening of the C-N bond lengths, (ii) the increase of the C(1) and C(8) ipso angles and the decrease of the N(1)-C(1)-C(9) and N(2)C(8)-C(9) angles, and (iii) the shortening of the C(3)-C(4) and C(5)-C(6) bond lengths. The most characteristic features of the anion are the relatively short localized C(2a)dC(3a) and C(5a)dC(6a) double bonds and different electronic effects of the substituents on C(1a) and C(4a) ipso angles. The -C(CN)2 group decreases the C(1a) ipso angle up to 116.6(3)°. The structure of DMANH+pCNPhN-, together with the structures of other ionic complexes of DMAN, leads to relationships between structural and spectroscopic results. Thus, Figure 9a shows a relation between the chemical shift of the acidic proton and the donor-proton N-H distance. Because the hydrogen bonds in the proton sponges are strong, there is a similar relation with the hydrogen-acceptor H‚‚‚N distance (Figure 9b). We also note the dependence confirming crosspolarization of the entire DMANH+ cation (Figure 9c). The
11426 J. Phys. Chem., Vol. 100, No. 27, 1996 changes of the N-H-N angle resulting from the interactions of the [N-H‚‚‚N]+ hydrogen bond with the nearest environment are related to the average changes of the 13C MAS NMR chemical shifts of atoms C(3) and C(6) located on the other side of the cation. Evidently, the decrease of the N-H-N angle (weaker minor interactions with the moieties outside the cation) is associated with the increase of the chemical shifts of the C(3) and C(6) atoms. Figure 9d reveals a correlation between the average chemical shifts of methyl carbon nuclei and the difference between binding energies of the donor and acceptor atoms (delta). This shows that the changes of the methyl groups are also important when the consequences of the formation of the [N-H‚‚‚N]+ hydrogen bond are considered. We conclude that strong ionic intramolecular [N-H‚‚‚N]+ hydrogen bonds do affect the core binding energies of nitrogen atoms and that, despite being referred to a non-hydrogen-bonded system, the ESCA chemical shifts between the binding energies of the donor and the acceptor atoms are a useful measure of the strength of the hydrogen bonds. The trends between the structural and spectroscopic parameters, found in the proton sponge complexes, are weaker than in perisubstituted compounds. Acknowledgment. We are grateful to the Royal Society for a Research Fellowship for K.W. and to the Department of Chemistry of the University of Warsaw, for financial support from Project 12-501/III/BST-502/24/95. Supporting Information Available: Atom coordinates, equivalent/isotropic temperature factors, structural parameters, and anisotropic thermal parameters (7 pages); tables of observed and calculated structure factors (5 pages). Ordering information is given on any current masthead page. References and Notes (1) Practical Surface Analysis, 2nd ed.; Briggs, D., Seah, M. P., Eds.; John Wiley: New York, 1980. (2) Barr, T. L. Modern ESCA; CRC: Boca Raton, FL, 1994. (3) Wagner, C. D.; Riggs, W. M.; Davis, L. E.; Moulder, J. F.: Muilenberg, G. E. Handbook of X-Ray Photoelectron Spectroscopy; PerkinElmer Corp.: Minnesota, 1979. (4) Wozniak, K.; He, H.; Klinowski, J.; Jones, W.; Barr, T. L. J. Phys. Chem. 1995, 99, 14667. (5) Wozniak, K.; He, H.; Klinowski, J.; Barr, T. L.; Hardcastle, S. J. Phys. Chem. 1996, 100, 11408. (6) Staab, H. A.; Saupe, T. Angew. Chem., Int. Ed. Engl. 1988, 27, 865.
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