Intramolecular Hydrogen Bonding and Molecular Geometry of 2

Intramolecular Hydrogen Bonding and Molecular Geometry of 2-Nitrophenol from a Joint. Gas-Phase Electron Diffraction and ab Initio Molecular Orbital ...
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J . Phys. Chem. 1994, 98, 1442-1448

1442

Intramolecular Hydrogen Bonding and Molecular Geometry of 2-Nitrophenol from a Joint Gas-Phase Electron Diffraction and ab Initio Molecular Orbital Investigation Konstantin B. Borisenko,la Charles W. Bock,*Jband Istvhn Hargittai'J' Institute of General and Analytical Chemistry, Budapest Technical University and Structural Chemistry Research Group of the Hungarian Academy of Sciences, H-1521 Budapest, Hungary, Chemistry Department, Philadelphia College of Textiles and Science, Philadelphia, Pennsylvania 191 44, and American Research Institute, Materials Science Division, Marcus Hook, Pennsylvania I9061 Received: October 8, 1993"

The molecular geometry of 2-nitrophenol has been determined by a joint investigation of gas-phase electron diffraction and a b initio molecular orbital calculations. R H F / 6 - 3 1G*, R H F / 6 - 3 1G**, and MP2/6-3 l G * optimizations were used to determine small parameter differences, such as A(N=O), A(C-C), and A(CN=O), which in turn were utilized as constraints in an electron diffraction structure analysis. The present experimental and calculated geometries are consistent regarding (i) the planarity of the molecule and (ii) all the structural features including strong hydrogen bonding between the nitro group oxygen and hydroxy hydrogen and the structural changes in the rest of the molecule as compared with phenol and nitrobenzene on the one hand and 2-nitroresorcinol on the other. Whereas the structural changes in 2-nitrophenol are less pronounced than those in 2-nitroresorcinol, they a r e consistent with a resonance form that implies the hydrogen bond and some redistribution of electron density in the benzene ring. The (N=)O-H(-O) and (N=)O-SO(-H) nonbonded distances are 1.72 f 0.02 and 2.58 f 0.01 A, respectively, and they do not differ, within experimental error, from those of 2-nitroresorcinol. The molecular geometry is characterized by the following bond lengths (rg) and angles, obtained in the electron diffraction analysis incorporating the constraints from the molecular orbital calculations: (C-H)mWn, 1.089 f 0.007 A; (C-C)man, 1.399 f 0.003 A; Cl-Cz, 1.41 1 f 0.012 A; c2-c3,1.406 f 0.013 A; c3-c4, 1.388 f 0.021 A; c4-c5, 1.399 f 0.027 A; C5-Cal 1.387 f 0.020 A; Ci-Ca, 1.402 f 0.016 A; C-0, 1.359 f 0.009 A; 0 - H , 0.969 f 0.012 A; C-N, 1.464 f 0.005 A; (N=O),,,, 1.233 f 0.003 A; N=014, 1.241 f 0.009 A; N=015, 1.225 f 0.009 A; LC6-C1-CZ1 121.4 f 0.5'; LCl-C2-C3, 119.4 f 0.8'; LC2-C3-c& 118.1 f 1.6'; LC3-C4-CS9 122.9 f 0.9'; &-C5-C6, 119.3 f 0.8', LCl-C6-C5, 119.0 f 0.8'; LCi-C*-O, 123.9 f 0.8'; LC-0-H, 104.4 f 2.2'; LCZ-CI-N, 120.8 f 0.7'; (LC-N=O),,n, 118.4 f 0.3'; LC-N=O,~, 118.2 f 1.0'; LC-N=015, 118.6 f 1.0'; LO=N=O, 123.3 f 0.4'; nitro group torsion, 7.3 f 5.7'.

Introduction Our recent gas-phase electron diffraction determination of the molecular structure of 2-nitroresorcinolz found indications of strong intramolecular hydrogen bonding, as well as considerable changes in the overall structure of the molecule as compared to the structures of phenol3 and nitrobenzene.4 These results have been confirmed by a recent investigation of 2-nitroresorcinol, phenol, and nitrobenzene using ab initio molecular orbital (MO) calculations.5 As a continuation of our research into orthosubstituted benzene derivatives, in which we are probing possible intersubstituent effects,2+5+* we have initiated the present investigationof 2-nitrophenol. The determination of the structure of this molecule is a more difficult challenge for gas-phase electron diffraction than 2-nitroresorcinol because of its lower symmetry. Consequently we decided to carry out ab initio MO calculations in conjunction with the experimental analysis, so that computed differences between certain parameters could be used as constraints in the analysis of the electron diffraction data. The gas-phase structure of 2-nitrophenol has been investigated previously using microwave spectroscopy.9a The molecule was found to be planar with an inertia defect A = -0.360 f 0.006 amu AZdue to nitro group torsion. Only a limited amount of additional geometrical information could be deduced from the microwave spectra due to the small number of available isotopomers. Some lengthening of the 0-H bond was indicated, but no marked structural effect was observed that could be ascribed to strong intramolecular hydrogen bond formation. Recently, the micro*Abstract published in Advance ACS Abstracts, January 1 , 1994.

0022-3654/94/2098- 1442%04.50/0

wave spectra of nitrobenzeneand o-nitrophenolhave been observed with high resolution using a wave guide microwave Fourier transform ~pectrometer.~~ In any case, the experimental rotational constants from the microwave study provided useful additional information against which the results of our combined electron diffraction and ab initio MO analysis could be tested. The molecular structure of 2-nitrophenol has also been investigated by X-ray crystallography.1o Apparently, the authors of this work were unaware of the prior microwave study. The geometrical parameters obtained in this X-ray study were corrected for thermal motion, but standard deviations were reported only for the uncorrected values. The (N=)O-O(-H) distance was found to be 2.602(6) A, suggesting considerable hydrogen bonding. Interestingly, the structure features of the remainder of the molecule indicated a remarkable contribution of an o-quinonoid structure to the benzene ring.

Ab Initio MO Computations Ab initio molecular orbital calculations on 2-nitrophenol were performed with the GAUSSIAN 92 series of programs," using the 6-3 lG* and 6-3 1G** basis sets.12 Initially, RHF/6-3 1G* and RHF/6-3 1G** optimizations were performed, followed by second-order Mdler-Plesset optimizations13with only the valence orbital active using the 6-31G* basis set, Le., MP2(FC)/6-31G* optimization. A vibrational frequency analysis was carried out at the RHF/6-31G*//RHF/6-31G* level to confirm that the computed completely planar structure was indeed a stable state. Analogous calculation on phenol, nitrobenzene, and 2-nitroresorcinol can be found in ref 5 . 0 1994 American Chemical Society

Bonding and Geometry of 2-Nitrophenol TABLE 1: 2-Nitrophenok Computed Geometries' parameter RHF/6-31G** RHF/6-31G* MPZ(FC)/6-31G* 1.2080 1.2549 N-14 1.2082 1.1883 1.2387 N-15 1.1883 C-N 1.4442 1.4443 1.4563 0-H 0.9499 0.9537 0.9855 C-O 1.3231 1.3245 1.3510 1.3932 1.4104 c1-c2 1.3984 1.3990 1.4046 c2-c3 1.3990 1.3713 1.3870 CFC4 1.3710 1.4021 1.3979 1.3981 C4-C5 1.3862 1.3683 1.3686 CYc6 1.3986 1.4009 c641 1.3983 C3-H 1.0734 1.0732 1.0863 C4-H 1.0755 1.0751 1.0871 C5-H 1.0736 1.0733 1.0860 Cs-H 1.0713 1.0708 1.0844 LO=N-O 123.13 123.15 122.80 LC-N-4 118.25 118.24 118.37 L C - N ~ I J 118.63 118.61 118.83 LN-CI-C~ 121.24 121.31 120.98 LCl-CrO 125.70 125.82 125.60 LC-0-H 110.28 110.27 106.78 Lc6-CI-c2 121.18 121.14 121.65 L C I - C ~ C ~ 117.60 117.63 117.44 120.83 121.12 LC#&C4 120.84 Lc3-cd-C~ 12 1.18 121.15 120.53 Lc4-CJ-Cs 118.99 118.98 119.69 LCJ-C6-c1 120.23 120.27 119.58 LCrC3-H 117.45 117.45 117.22 LC3-C4-H 119.13 119.15 119.38 LC4-C5-H 120.67 120.67 120.53 LCyC6-H 121.27 121.20 121.74 energy (au) -509.048701 -509.034670 -5 10.506869 "ren equilibrium bond lengths (A) and angles (deg). TABLE 2 Exwrimental Conditions for 2-Nitrophenol camera nozzle data data no. of dist, temp, wavelength, intervals, steps, plates mm OC A A-l '4-1 8 190.6 103 0.049 23 9.75-36.00 0.25 8 499.8 103 0.04923 1.875-14.000 0.125 The computed equilibrium geometries and total molecular energies of 2-nitrophenol are listed in Table 1. The geometrical parameters computed a t the RHF/6-3 lG* and RHF/6-31G** levels are nearly the same. Including the correlation effect at the MP2( FC)/6-3 lG*//MP2( FC)/6-3 1G* level generally increases the bond lengths but decreases the length of the hydrogen bond. Of the bond angles, only the C-0-H angle appears sensitive to including the effects of electron correlation. All these observations are consistent with those made on the computed results for phenol, nitrobenzene, and 2-nitrore~orcinol.~ We may add here that the MP2(FULL)/6-31G* optimization for phenol and nitrobenzene showed only negligible differences from the MP2(FC)/6-31G* optimizations.$ We note also that the general pattern of the 2-nitrophenol molecular geometry obtained in the present investigation is in agreement with that of an earlier study at the RHF/3-21G computational level,14 and various features of its structure had also been reported from semiempirical calculations.lS Electron Diffraction Experiment A commercial sample of 2-nitrophenol (Polskie odczynniki chemiczne Gliwice distributed by Reanal) was used in our electron diffraction study following a purity check by liquid chromatography. The electron diffraction photographs were recorded in our modified EG-lOOA apparatus16awith a membrane nozzle Some of the experimental conditions are summarized in Table 2. The electron scattering factors were taken from available compilation^.^^ The experimental and theoretical molecular intensities and radial distributions are shown in Figures 1 and 2. The numbering of atoms is presented in Figure 3.

The Journal of Physical Chemistry, Vol. 98, No. 5, 1994 1443

2-Nitrophenol

50cm

0

2A

5

10

-

15

20

25,,~'30

-

35

Figure 1. Experimental (E) and theoretical (T) molecular intensities and twice their differences (2A) for 2-nitrophenol.

2-Nitrophenol

/I

1

~

E

2A v

I

-

I

0

1

2

3

4 r,A

6

Figure 2. Experimental (E) and theoretical (T) radial distributions and twice their difference (2A) for 2-nitrophenol.

10

Figure 3. Numbering of atoms for the series phenol, nitrobenzene, and 2-nitrophenol.

Structure Analysis A preliminary analysis was carried out using only the experimental electron diffraction data and applying the leastsquares method to the molecular intensities.18 The molecule was generally assumed to have C, symmetry except in cases where a torsional twist of the substituent groups was being investigated. Eventually, however, constraints have been introduced from the ab initio MP2(FC)/6-31G* calculations, as described below. Initially the nitro group was taken to have CZ,symmetry with the symmetry axis along the C-N bond. Eventually, this contraint was eliminated as differences between the two N=O bonds and between the two C-N=O angles were incorporated into the analysis. Refinements allowing for an N=O bond length difference have always shown an increase in the N=O bond length participating in hydrogen bonding and a decrease of the other N=O bond length. Using only the electron diffraction data, this difference refined to 0.027(7) A. However, at the final stages of our overall analysis this difference was fixed at a value of 0.016 A, which was obtained from the a b initio MP2(FC)/ 6-3 l G * optimization. Incidentally, these twovalues did not differ beyond the experimental error, since the estimated total error for

1444

The Journal of Physical Chemistry, Vol. 98, No. 5, 1994

SCHEME 1

A N 0 was 0.017 A in the final least-squares refinement of the combined experimental and ab initio molecular orbital data. The difference between the two C-N=O angles was also carefully examined. In the X-ray crystallography study of 2-nitrophen01,lo the C-N-0 angle participating in the sixmembered ring with hydrogen bonding was found to be smaller by a few tenths of a degree than the other C-N-0 angle. Ab initio calculations at the MP2(FC)/6-31G* level have indicated a small difference of the same sign. However, our electron diffraction least-squares refinements of 2-nitrophenol have yielded a larger value for the C-N=Oangle participating in the hydrogen bonding than the other angle. A series of refinements was carried out with the difference between the two C-N=O angles fixed at 0.0, fixed at the value obtained from the ab initio MP2(FC)/ 6-31G* calculations, and varied as an independent parameter. Although these refinements have resulted in different values of the C-N=O angles, other parameters remained the same within their experimental errors. At the final stage of the analysis, incorporating the electron diffraction data and the constraints from the ab initio calculations, we also probed the possibility of nitro group torsion around the C-N bond. Introduction of such a torsional twist did not improve the agreement between the model and the experimental data. Its value refined to 7.3(40)', which is consistent with small torsional vibrations of the nitro group around the planar equilibrium conformation. The a b initio calculations themselves have also predicted the structure to be planar. The hydroxy group was described with a ACO bond length difference between the C-C bond in between the substituents and the C-0 bond, with a AOH difference between (C-H)man and the 0-H bond. In addition, the C-0-H angle was refined as an independent parameter. Unfortunately, the angle of torsion around the C-0 bond could not be obtained in the least-squares refinements. Thus it was fixed to zero as suggested by the ab initio calculations (vide supra). This value permits the closest contact between the nitro group oxygen and hydroxy group hydrogen. The benzene ring geometry was represented by the following independent parameters in the joint analysis: C& bond length in between the nitro and hydroxy groups, four differences between the C1-C2 bond and other C-C bond lengths, and four C-C-C angles. All C-H bonds were assumed to be of the same length and directed along the bisectors of the respective C-C-C angles. Considering the results of the electron diffraction study of 2-nitroresorcinol~there should be an appreciable shortening of the and c 5 - C ~ bonds with respect to the C-C bond in between the substituents and, to a lesser extent, with respect to theother C-C bonds. Thisis apparentlya result of the importance of the a-quinonoid resonance form, shown in Scheme 1, to the overall structure of 2-nitrophenol. As in the case of 2-nitroresorcinol,2 intramolecular hydrogen bonding should be greatly assisted by resonance in 2-nitrophenol as well. However, attempts to distinguish the C-C bond lengths by varying their differences in the least-squares refinements involving only the electron diffraction data have always led to a structure in which all C-C bond lengths, except the one between the nitro and hydroxy groups, were the same within their, admittedly large, standard deviations.

Borisenko et al. Constraining all the C-C bond length differences to the same value caused the precision in determination of the mean value of the corresponding four C-C bond lengths to increase considerably, while the other parameters were not markedly influenced. It should be noted, however, that the a b inition MP2(FC)/6-31G* calculations indicate a considerable split in the C-C bond lengths of the benzene ring in accordance with the a-quinonoid resonance structure of Scheme 1. Thus it was decided to assume the calculated differences in the final refinements of the parameters as part of the joint analysis. As in the case of 2-nitroresorcinol, there was a shift at the 4.8-Amaximum toward smaller valueson theexperimental radial distribution as compared with the theoretical distributions. Distances contributing to this maximum, viz., Cp015 and C5-014, were varied as independent parameters. This proved sufficient to eliminate the discrepancy. Introduction of these nonbonded distances into the refinement as independent variables did not influence the results of the refinement to any appreciable extent. The mean amplitudes of vibration were grouped together according to the appearance of the contributions of the corresponding distances on the radial distribution. The differences among them, within the same group belonging to the same maximum, were kept fixed in the refinements. The amplitudes of nonbonded C-.C benzene ring distances were also fixed at the values obtained in the electron diffraction study of 2-nitroresorcinol.2 We have tested the influence of these assumptions on the other parameters and found that the respective changes did not exceed experimental error. Finally, the geometry of the joint scheme was described by bond lengths, bond length differences, bond angles, and angles of torsion, as well as by the two nonbonded distances mentioned above. They are all indicated as independent parameters in Table 3 with the ab initio constraints marked. The main results of the least-squares refinements are also presented there. They can be considered the final results of our combined analysis. It is important to stress that the principal parameters have proved to be practically insensitive to changes in the refinement conditions. Table 4 lists the elements of the correlation matrix that exceed 0.6 in absolute value. The rotational constants calculated from the geometrical parameters of Table 3 are also given in Table 3. The rotational constants determined from the microwave spectra9for the normal species are shown as well. The agreement is even better (within a couple of tenths of a percent) than can be expected given the difference in the physical contents of these two sets of rotational constants. Throughout the analysis, the rotational constants have served as a gauge of the general consistency of the structures. However, they were not included in the refinement because, lacking vibrational corrections, this might have introduced some systematic error in the analysis. It is our general observation that the rotational constants of 2-nitrophenol are rather insensitive to reasonable geometrical changes in our molecular model. Drastic changes in the model, however, cause considerable changes in the rotational constants. Thus, for example introducing a 180' torsion about the C - O bond changesoneoftherotationalconstants by 2% ( A , 2322(4); B, 1277(6); and C,825(2) MHz).

Results and Discussion The bond lengths (rg),bond angles, and the angle of the nitro group torsion from the electron diffraction analysis incorporating constraints from MP2(FC)/6-31G* ab initio calculations with estimated total errors are listed in Table 5 . Both systematic errors and least-squares standard deviations were included in the estimated total errors.19 The least-squares standard deviations of Table 3 were obtained in a special calculation in which the parameter differences assumed from the ab initio results were also treated as variables. Those parameters of 2-nitrophenol that

Bonding and Geometry of 2-Nitrophenol

The Journal of Physical Chemistry, Vol. 98, No. 5, 1994 1445

TABLE 3 Results of Electron Diffraction Least-Squares Refinement' Incorporating Constraints (in Bold) from MP2( FC)/ 6-31C* ab Initio Calculations

1.410(8) 1.084(4) O.OO!%d (1 52) 0.0234d (171) 0.0099 (184) 0.0242d (87) 0.052(10) 0.120(8) 0.052(9) 0.170( 11) 0.01v (12) -0.4d (1 3)

0.049(1) 0.078

1.404(9) 1.386(15) 1.397(19) 1.385(14) 1.400(11) 1.397(1) 1.358(6) 1.462(3) 1.239(6) 1.223(6) 1.231(1) 0.963(8) 2.429( 12) 2.400(6) 2.748(4) 2.392(9) 2.450(9) 2.805(11) 2.444(8) 2.827(8) 2.401( 13) 2.442(6) 2.320(8) 2.312(6) 2.775(8) 3.587(10) 2.35 l(7) 4.175(11) 4.738(14) 3.6 12(8) 4.93 l(9) 4.900( 8) 4.162(7) 4.715(11) 4.093(14) 3.7 12(8) 3.564( 15) 2.710(11) 2.497(7) 3.775 ( 10) 4.210(4) 3.723(7) 2.452(8) 2.918(6) 2.577 (9) 4.133(6) 2.167(2) 2.349(23) 4.662( 12) 5.293(5) 4.597(9)

0.049 0.049 0.049 0.049 0.049

121.4(3) 119.4(5) 118.1(11) 119.0(6) 123.9(5) 104.4(16) 120.8(4) 118.4(1) 0.W 7.3(40) 4.688(7) 4.71 l(7) i i i i

0.046 0.052 0.043 0.043

i i

0.078 0.0547c 0.0547c 0.0598' 0.0547' 0.0547' 0.0598c 0.0547' 0.0598* 0.0547 0.052(2) 0.058 0.058 0.086(7) 0.073(5) 0.054 0.094(2)

i

0.061 0.079 0.079 0.060 0.094 0.060 0.073 0.086 0.057 0.067 0.068 0.067 0.057 0.079 0.094 0.093 0.049 0.157 0.130 0.125 0.130

2.657( 10) 2.587(12) 4.477(11) 5.245(8) 4.598(9) 1.713( 14) 4.859(14) 5.996(10) 5.672( 13) 3.884( 14) 3.548(23) 5.707( 15) 5.958(9) 4.757( 15) 2.401 (14) 2.394(25) 3.419(15) 3.832(6) 3.389(8) 2.162(11) 1.849(22) 2.170(14) 3.373( 10) 3.889( 10) 3.43 l(9) 3.122(16) 2.129( 12) 3.417(9) 3.9 11(10) 4.241 (20) 2.154( 19) 2.1 58( 19) 3.390( 15) 4.526(24) 3.423(9) 2.139(20) 2.149( 15) 3.785(26) 3.911(10) 3.375(15) 2.147( 13) 122.9(6) 119.3(6) 118.2(7) 118.6(7) 123.3(2) 1.5(4) 3.6(5) 147.3(22) 104.3(11) 2.73%

i

1 1

iii iii iii iv V

iii

vi V 11

ii vi vi V V

iv

iii V

vi V

iii iv

iii vi

iii iii ii I1

ii

microwavespectroscopy' A B

electron diffractiod

1

2

2365.9(50) 1275.3(24)

2368.738(50) 1276.1 lO(6)

2368.7821(50) 1276.1327(9)

0.072(3) 0.072 0.117 0.140 0.138 0.1 16 0.138 0.087 0.169 0.189 1f 0.1891f 0.141 0.163 0.1891f 0.1891f 0.169 0.142 0.146 0.107 0.110 0.107 0.091 0.105 0.106 0.107 0.107 0.110 0.108 0.106 0.107 0.1 10 0.119 0.106 0.106 0.110 0.161 0.107 0.106 0.106 0.157 0.107 0.107 0.106

ii ii iv

iii ii ii ii i ii V V

ii iii iii V

V V

iii iii iii V V V V

iii V

V

vi iii

iii V

ii V

iii iii V V

V

iii

microwavespectroscopy&

c

electron diffractiod

1

2

830.0(6)

829.812(7)

829.8260(9)

Least-squares standard deviations are parenthesized in units of the last digit. ACC1 = r ( c 1 - C ~-r(CrC3), ) ACC2 = r(c1-C~)-r(C3-C4), ACC3 = r(c1-C~)- r(Cl-C& ACC4 = r ( c 1 - C ~-) r(CS-c6), ACO = r ( C 4 2 ) - r(C-O), AOH = r(C-H)man - r(O-H), ACN = r(C-N) - r(CI-Cz), A N 0 = r(c1-C~)- r ( N 4 1 4 ) . AN02 = r(N=014) - r(N=015), ACNO = LC-N=O~~- LC-N=O15. e Angles of torsion, around the C-N bond, 4, and around the C-0 bond, 7. Assumed from the ab initio calculations (bold, present work). Assumed from the analysis of 2-nitroresorcinol, ref 2. /Assumed. Positive when tilt is away from the hydrogen bond. Angle made by the 0-H bond and the hydrogen bond. Angle made by the hydrogen

*

bond and the N=O bond. Calculated from the above geometry. Experimental values for the normal species from the microwave spectroscopic investigations, refs 9a (1) and 9b (2).

1446 The Journal of Physical Chemistry, Vol. 98, No. 5, 1994 TABLE 4 Correlation Matrix Elements with Absolute Values Greater Than 0.6 for 2-Nitrophenol i ~(CI-C~)

LC-CI-C LC-Cz-C

LC1-Cz-0 ACO scale 50 cm

i

xii

ACN AN0 LC-Cz-c LC1-CrO LC2-C I-N LC-C3-C LC1-C2-0 LC2-CI-N LC-C3-C LCz-CI-N AN0 4C-H)mean

-0.6997 0.9162 -0.7486 0.7275 -0.7242 -0.7662 -0.7207 0.6533 0.7906 -0.8034 0.6536 -0.6663

TABLE 5 Bond Lengths ( r A), Bond Angles, and the Angle of Torsion (deg) of 2-fitrophenol with Estimated Total Errors from the Electron Diffraction Analysis Incorporating Constraints from MP2(FC)/6-31G* ab Initio Calculations 121.4 f 0.5 1.411 0.012 LC6-C1-CZ G-C2 LCI-CZ-C~ 119.4 f 0.8 CZ-C3 1.406 f 0.013 LC&3-C4 118.1 f 1.6 c3-c4 1.388 f 0.021 122.9 f 0.9 1.399 f 0.027 LC~-C~-CS C4-Cs 119.3 f 0.8 1.387 f 0.020 Lc4-c5-c6 c5-c6 LCI-CS-CS 119.0 f 0.8 1.402 f 0.016 CI-CO 123.9 f 0.8 1.399 f 0.003 LCl-Cz-0 (C-C)mean LC-0-H 104.4 f 2.2 1.089 f 0.007 (C-H)mMn LCrCl-N 120.8 0.7 c-0 1.359 f 0.009 L C - N ~ I ~ 118.2 f 1.0 0-H 0.969 f 0.012 LC-N=Oij 118.6f 1.0 C-N 1.464 0.005 N414 1.241 0.009 (LC-N=O)m,, 118.4 f 0.3 N=Ols 1.225 0.009 LO=N=O 123.3 f 0.4 (N=O),,, 1.233 f 0.003 6' 7.3 f 5.7 The angle of the nitro group torsion.

*

* *

are not listed in Table 5 but appear in subsequent tables are also quoted with estimated total errors (e.g., angles of tilt). Nitro Group Torsion. 2-Nitrophenol was found planar by microwave spectros~opy.~ However, torsional vibrations of the nitro group have resulted in a small negative deviation of the inertia defect, A = -0.360 f 0.006 amu A*, from the ideal zero value for rigid planar molecules. The angle of the nitro group torsion was found to be 7.3 f 5.7O in the present electron diffraction study of 2-nitrophenol. Nevertheless, the molecule could be considered to possess a planar equilibrium conformation. Marginal deviations from planarity, as found by electron diffraction, may be due to the nitro group torsional vibrations and are thus ascribed to an operational effect. The electron diffraction structure corresponds to an inertia defect of A = -1 .O f 0.6 amu Az and, conversely, the inertia defect from the microwave spectra may be estimated to correspond to an angle of nitro group torsion of 4.4 f 0.5'. There is consistency between these data from the two techniques taking the large experimental errors of the electron diffraction analysis into account. We note also that there is no intrinsic requirement to suppose that a rigorously planar structure should be favored. It is stressed, however, that the RHF/6-31G*//RHF/6-3 lG* frequencyanalysis found the completely planar 2-nitrophenol molecule to be stable. Thus we may assume a planar equilibrium conformation for 2-nitrophenol. Supposing such a planar equilibrium geometry, the effective torsional angle obtained in the least-squares refinement suggests a barrier to nitro group torsion in between those of nitrobenzene4 and 2-nitroresorcinol.2 The findings concerning the close to zero torsion of the nitro group from the plane of the ring are at variance with a report on 170 N M R spectroscopic investigation of intramolecular hydrogen bonding for o-nitrophenols.20 The N M R data were interpreted to be consistent with MM2 calculations of the geometry of 2-nitrophenol, characterized by a 1.84 8, (N=O)O-.H(-0) distance and a 21° angle of nitro group torsion.21. Intramolecular Hydrogen Bonding. A prominent contribution

Borisenko et al.

TABLE 6 Comparison of Phenol and 2-Nitrophenol Geometrical Parameters from Electron Diffraction and MO Calculations MO calculationsC electron diffraction MP2(FC)/ 6-31G* parameter phenol' 2-nitrophenolb difference difference Cl-cz, A 1.411(12) +0.012 +0.014 CrC3, A 1'399(3) 1.406(13) +0.007 +0.007 0-H, A 0.958(3) 0.969(12) +0.011 +0.012 c-0, A 1.381(4) 1.359(9) -0,022 -0.024 LC-O-H, deg 106.4(17) 104.4(22) -2.0 -1.5 LO-Cz-Cl, deg 121.2(12) 123.9(8) +2.7 +2.8 119.4(8) LC-CrC, deg 121.6(2) -2.2 -2.9 CO, tilt, deg +2(1) +3.6(7) +1.6 +1.3 Reference 3. From a joint analysis incorporatingconstraints from an MP2(FC)/6-31GS ab initio calculations, present work. Phenol, ref 5 ; 2-Nitrophenol, present work. TABLE 7: Comparison of Nitrobenzene and 2-Nitrophenol Geometrical Parameters from Electron Diffraction and MO Calculations MO calculationsc electron diffraction MP2(FC)/ -3 iG* ' nitroparameter benzenea 2-nitrophenolb difference difference +0.011 +0.018 c1-c2, A 1.400(3) 1.411(12) +0.006 cI-c6, A 1.396(3) 1.402(16) +0.008 1.241(9) +0.018 +0.013 N414' A 1.223(3) 1.225(9) N=Ois, A +0.002 -0.004 1.486(4) 1.464(5) C-N, & . -0.022 -0.015 LC-N+14, deg 117.3(1) 118.2(10) +0.9 +0.7 118.6(10) LC-N=O~S,deg +1.3 +l.2 LO=N=O, deg 125.3(2) -2.0 123.3(4) -1.9 LN-CI-C~, deg 118.3(3) 120.8(7) +2.5 +2.4 117.8(7) LN-CIX6, deg -0.5 -1.2 123.4(3) 121.4(5) LC-CI-C, deg -2.0 -1.1 CN, tilt, deg +1.5 + 1.5(6) +1.8 'Reference 4. From a joint analysisincorporatingconstraints from an MP2(FC)/6-3 1G*abinitiocalculation,present work. Nitrobenzene, ref 5 ; 2-Nitrophenol, present work. '

of theo-quinonoid resonance form to the structureof 2-nitrophenol is supposed to occur if this form is stabilized by hydrogen bonding. On the other hand, as a consequence of hydrogen bond formation, a redistribution of electron density in the molecule should lead to a structure represented by the o-quinonoid form of Scheme 1. Actually, the results of the joint analysis on the bond lengths in the benzene ring as compared with phenol' and nitrobenzene4 are consistent with the expected consequences of hydrogen bond formation. Comparison of 2-nitrophenol with phenol, nitrobenzene, and 2-nitroresorcinol both from the joint analysis and from the ab initio MO calculations alone is presented in Tables 6, I , and 8. The distance of the nonbonded interaction (N=)O-H(-0) and the nonbonded (N=)O-0(-H) distance are in agreement, within experimental errors, in 2-nitrophenol and 2-nitroresorcinol (Table 8). This suggests about equal hydrogen bond strength in the two molecules. There is a lengthening of the 0-H bond in 2-nitrophenol, by 0.01 A, as compared with phenol (Table 6). The data of the joint analysis and the M O computational data a r e fully consistent on this change.23 It is interesting to note that the C-0 bond in 2-nitrophenol and 2-nitroresorcinol is of about the same length. On the other hand, there are marked changes in the parameters associated with the nitro group and adjacent moiety of the molecule in the series nitrobenzene, 2-nitrophenol, and 2-nitroresorcinol. These changes inr(C-N),r(Cl-C2), LC-Cl-C, andLO=N=Oof 2-nitrophenol, as compared with nitrobenzene, amount to nearly half of those

The Journal of Physical Chemistry, Vol. 98, No. 5, 1994 1447

Bonding and Geometry of 2-Nitrophenol

TABLE 8: Comparison of 2-Nitrophenol and 2-Nitroresorcinol Geometrical Parameters from Electron Diffraction and MO Calculations MO calculationsc electron diffraction MP2(FC)/ 2-nitro- 2-nitro6-3 1G* phenol' resorcinolb difference difference

parameter CrC2, A 1.411(12) CZ-CI, A 1'406(13) CrC4, A 1.388(21) N=Ol4, A 1.241(9) N=Ols, A 1.225(9) C-N, A 1.464(5) C-0, A 1.359(9) 0-H, A 0.969(12) ~C-N=014,deg 118.2(10) LC-N=015, deg 118.6(10) L O = N = ~ , deg 123.3(4) LC-O-H, deg 104.4(22) LN-C~-C~.deg 120.8(7) LO-C&, deg 123.9(8) LC-CI-C, deg 121.4(5) LC-CZ-C, deg 119.4(8) CO, tilt, deg +3.6(7) (N=)O*-H(-O), A 1.72(2) (N=)O-O(-H), A 2.58(1) LN=O.-H, deg 104.3(15) LO-H-O, deg 147(3) a

1.426(5) 1.393(4) 1.239(3) 1.449(7) 1.354(4) 1.038(15) 119.3(3)

+0.015 -0013

+0.015

-0002 +d.014 -0.015

-0.002 +o,o14 -0,017 0.000 -0.001 +1.4 +0.9 -2.3 +0.8 -1.1 -0.9 -1.5 +l.5 -0.1 -0.032 -0.031 -0.2 +0.1

+,joo5-0.010 +o.oo3

-0.005 +0.069 +1.1

+0.7 121.4(5) 116(3) 120.5(4) 122.8(7) 119.1(7) 120.4(5) +2.9(5) 1.76(4) 2.56(1) 110.5(15) 131(5)

-1.9 +11.6 -0.3 -1.1 -2.3 +1.0

-0.7 +0.04 -0.02 +6.2 -16

From a joint analysisincorporatingconstraints from an MP2(FC)/

6-31G' ab initio calculations, present work. Reference 2. 2-Nitroresorcinol, ref 5 ; 2-nitropheno1, present work.

SCHEME 2

in the analogous parameters of 2-nitroresorcinol. Since the magnitude of change directly correlates with the number of hydrogen bonds in which the nitro group is involved in the molecule, there seems to be near additivity in the geometrical consequences of hydrogen bonding. Comparing the resonance forms of 2-nitrophenol in Scheme 1 with those of 2-nitroresorcinol in Scheme 2 some further slight structural differences could be expected between 2-nitrophenol and 2-nitroresorcinol. According to these expected differences, the N=O bond in 2-nitrophenol participating in hydrogen bonding should be longer than the N-0 bonds of 2-nitroresorcinol. The C-0 bond in 2-nitrophenol should beshorter than the C-0 bonds in 2-nitroresorcinol. Furthermore, the C-C bond lengths outside the region between the two substituents should show a more pronounced alternation in 2-nitrophenol than in 2-nitroresorcinol. These expectations are based on the fact that 2-nitroresorcinol has two o-quinonoid resonance structures in each of which only one of the two hydroxy groups is involved. These two structures should be averaged, giving a structure of 2-nitroresorcinol possessing C2, symmetry. In this case, the N=O, C-0, CZ-C~, and C3-C4 bond lengths will be averaged from the two resonance forms, in which they are represented as formal double bond in one and formal single bond in the other. In contrast, 2-nitrophenol has an unique o-quinonoid resonance form only. As a result of averaging the two o-quinonoid resonance structures of 2-nitroresorcinol, one could expect those bonds that are represented as formal single bonds to be slightly shorter than the respective bonds in 2-nitrophenol, while those bonds that are represented as formal double bonds should be slightly longer than the respective bonds in 2-nitrophenol (see Scheme 1).

Unfortunately, our results do not appear sensitive enough to make any firm observation concerning the changes of the N=O and C-0 bond lengths as going from those involved in hydrogen bonding in 2-nitrophenol to those in 2-nitroresorcinol. On the other hand, the computed C-C bond length alternation outside the region between the substituents is indeed more pronounced in 2-nitrophenol thanin 2-nitroresorcinol. The electron diffraction data did not allow us to distinguish between these C-C bond lengths either. W e have noted the consequences of a repulsive intersubstituent effect between the nitro group and hydroxy group tending to expand the bond angles in 2-nitroresor~inol.~ There was also a small but characteristic tilt of the C-0 bond away from the hydrogen bond, 2.9 f 0.5'. This tilt was determined to be 3.6 f 0.7' in the joint analysis of 2-nitrophenol. In addition, there is a tilt of the C-N bond of 1.5 f 0.6', again, away from the hydrogen bond. The molecular symmetry, of course, precluded any tilt of the C-N bond in 2-nitroresorcinol (as well as in nitrobenzene). Whereas the C-N=O bond angle is greater in 2-nitrophenol than in nitrobenzene, the C-0-H angle is smaller than in phenol although the large error limits make the C-0-H angles, in fact, indistinguishable. Benzene Ring Geometry. The mean value of the C-C bond length in the benzene ring of 2-nitrophenol appears to be the same as in nitrobenzene4 and phenol,3 all 1.399 f 0.003 A. The mean C-C bond length of 2-nitroresorcinol2 is slightly greater, 1,404 f 0.003 A, but the difference is within experimental error. The mean C-H bond length in 2-nitrophenol, 1.089 f 0.007 A, is also in agreement with those in nitrobenzene4 (1.093 f 0.004 A), phenol3 (1.086 f 0.003 A), and 2-nitroresorcino12 (1.090 f 0.015 A). As was stressed above, the C-C bond alternation in the benzene ring of 2-nitrophenol is consistent with that implied by the o-quinonoid resonance structure of Scheme 1. Although these differences were accepted from ab initio MO calculations, there is direct experimental evidence of similar C-C bond length distribution in the benzene ring of 2-nitrophenol in the solid state as found by X-ray crystallography.1° Concerning the angular deformation, there is a marked departure from additivity of substituent impact.25 Both 2 4 troresorcinol and 2-nitrophenol demonstrate strong intersubstituent interaction. Therefore, additivity, based on the influence of independent substituents, is not valid. Assuming additivity, the following benzene ring angles (1/2) can be calculated for 2-nitrophenol from the electron diffraction structures of nitrobenzene4 and phenol' (1) and from the angular substituent parameters in thesolidstate (2):25C6C1C2,122.2/122.5'; CICzC3, 119.3/118.3'; C2C3C4, 119.3/119.9'; C3C4C5, 120.8/121.0'; C ~ C ~ C120.2/119.7'; S, c&&1, 118.3/118.7O. These two patterns are very similar, but neither the X-ray crystallographic structure of 2-nitrophenollo nor the present results are consistent with them. W e can add here that although the C-C bond length differences were assumed from the ab initio calculations, the benzene ring angle differences in 2-nitrophenol, as obtained in the electron diffraction analysis, did not reproduce those from the computations. Although there are relatively large experimental errors associated with these angles, the discrepancies go beyond them in some cases. Constraining the ring angle differences to the computed values led to a sharp increase in the R-factor while the other parameters proved insensitive to these changes.

Conclusions 1. There is strong, resonance-assisted intramolecular hydrogen bonding in 2-nitrophenol. 2. 2-Nitrophenol has a lower molecular symmetry than 2-nitroresorcinol, and electron diffraction alone could have yielded much less information about its geometry than the present joint electron d i f f r a c t i o n / a b initio M P 2 ( F C ) 6 - 3 l G * inves-

1448 The Journal of Physical Chemistry, Vol. 98, No. 5, 1994

Borisenko et al.

Relax. Inreract. Proc. 1978, 12,265. Rios, M. A.; Rodriguez, J. J. Comput. Chem. 1992,13, 860. (16) (a) Hargittai, I.; HernBdi, J.; Kolonits, M. Prib. Tekh. Eksp. 1972, 239. Hargittai, I.; Tremmel, J.; Kolonits, M. Hung. Sci. Insrrum. 1980,50, 31. (b) Hargittai, I.; Hernidi, J.; Kolonits, M.; Schultz, G. Rev. Sci. Insrrum. 1971, 42, 546. (17) (a) Coherent: Bonham, R. A.; Schifer, L. In International Tables for X-ray Crystallography;Kynoch Press: Birmingham, 197%Vol IV, Chapter 2.5. (b) Incoherent: Tavard, C.; Nicolas, D.; Rouault, M. J. Chim.Phys. Phys.-Chim: Eiol. 1967, 64, 540. (18) Andersen, B.; Seip, H. M.; Strand, T. G.; Stelevik, R. Acta Chem. Scand. 1969, 23, 3224. (19) Harnittai. M.; Harnittai, I. J. Chem. Phw. 1973. 59. 2513. (20j Boykn, D. W. J. Mol. Srrucr. 1993, 29j, 39. (21) Incidentally, 2-nitroresorcinol was also found to have considerable nitro group torsion (26O) in the quoted MM2 calculations,” while, curiously, Acknowledgment. W e thank Mrs. MBria Kolonits for expernitrobenzene itself was found to have zero torsion. These results on 2-nitroresorcinolare, again, at variance with our previous electron diffraction2 imental work, Dr.Jen’d Fekete for checking the purity of the and computational’ results. On the other hand, the planar structure of sample, and Dr. Vladiszlav Izvekov for advice. K.B.B. thanks nitrobenzene is in agreement with microwave,22 electron diffraction,’ and the J. Varga Foundation of the Budapest Technical University computational’ evidences. (22) Heg, J. H.; Nygaard, L.; Ssrensen, G. 0. J . Mol. Srrucr. 1970, 7 , for a Ph.D. Student Fellowship. This research has been supported 111. by the Hungarian National Scientific Research Foundation (23) There is no such consistency concerning the 0-H bond length change (OTKA, No. 2103). of 2-nitroresorcinol between the electron diffraction2 and computed’ results, as compared with phenol, +0.08 A versus +0.01 A. The bond length from electron diffraction? 1.038(15) A, seems to be exaggerated, although there Supplementary Material Available: Tables showing total is a large experimental error. We have now tested the experimental electron experimental electron diffraction intensities and background data diffraction data against models of 2-nitroresorcinol in which smaller 0-H ( 5 pages). Ordering information is given on any current masthead bond lengths were assumed. The R-factor is rather insensitive to a change in the assumed value of r(0-H) in a relatively broad range. Furthermore, page. the other parameters are rather insensitive to this change too. An example illustrates the situation. The (N=)O-.H(-O) distance is 1.76(4), 1.77(4), References and Notes 1.77(4), and 1.77(4) A at theassumed r(0-H) valuesof 1.022,1.012,1.002, and 0.992 A, respectively. According to a Hamilton test (Hamilton, W. C. (1) (a) Budapest Technical University and Hungarian Academy of StatisticsinPhysicalScience. The RonaldPres: New York, 1964),decreasing Sciences. (b) Philadelphia College of Textiles and Science and American the assumed value of r(0-H) to 1.002 A does not change the agreement Research Institute. within a 99.5% significance level. Thus, the relatively long 0-H bond of (2) Borisenko, K. B.; Hargittai, I. J . Phys. Chem. 1993, 97, 4080. 2-nitroresorcinolis, at least, suspect, although at this point there is noevidence (3) Portalone, G.; Schultz, G.; Domenicano,A.; Hargittai, I. Chem. Phys. to disprove it except that the quantum chemical calculations strongly suggest Lett. 1992, 197, 482. that its value is exaggerated. In this connection, it is of interest to examine (4) Domenicano, A,; Schultz,G.; Hargittai, I.; Colapietro, M.; Portalone, the O-H bond length change in monomer/dimer formic acid. An early electron G.; George, P.; Bock, C. W. Srruct. Chem. 1990, 1, 107. diffraction studyz4 reported 0.984(24) and 1.036(17) A for the 0-H bond (5) Bock, C. W.; Hargittai, I. To be published. length in the monomer and dimer, respectively; thus the difference was 0.05 (6) Vajda, E.; Hargittai, I. J . Phys. Chem. 1992, 96, 5843. A. We have now carried out a MPZ(FULL)/6-31G* optimization of these (7) Cdkviri, E.; Hargittai, I. J . Phys. Chem. 1992, 96, 5837. structures yielding 0.9801 and 0.9999 A, correspondingto a difference of 0.02 (8) Vajda, E.; Hargittai, I. J . Phys. Chem. 1993, 97, 70. A. Incidentally, the computations on 2-nitrophenoland 2-nitroresorcinolwere (9) (a) Leavell, S.;Curl, R. F., Jr. J . Mol. Spectrosc. 1973,45,428. (b) carriedoutat theMP2(FC)/6-31GS level. Theformicacidcalculationsshow Heineking, N., Dreizler, H. Eer. Eunsenges. Phys. Chem. 1993, 97, 663. no appreciable difference between the full and frozen core results. These (10) Iwasaki, F.; Kawano, Y. Acta Crysr. E 1978, 38, 1286. findings point to the desirability of the experimental reinvestigation of the (1 1) Frisch, A. M. J.; Trucks, G. W.; Head-Gordon, M.; Gill, P. M. W.; formic acid structures, as the change in the 0-H distance there seems to be Wong, M. W.; Foresman, J. B.; Johnson, B. G.; Schlegel, H. B.; Robb, M. exaggerated. The large experimental errors given in the early electron A.; Replogle, E. S.;Gomperts, R.; Andres, J. L.; Raghavachari. K.; Binkley, diffraction investigation allow, of course, a large range of difference to be J.S.;Gonzalez,C.;Martin,R.L.;Fox,D.J.;Defrees,D.J.;Baker,J.;Stewart, consistent with the experimental data. J. J. P.; Pople, J. A. Gaussian 92; Gaussian Inc.: Pittsburgh, PA, 1992. (24) Almenningen, A.; Bastiansen, 0.;Motzfeldt, T. Acta Chem. Scand. (12) Hehre, W. J.; Ditchfield, R.; Pople, J. A. J. Chem. Phys. 1972, 56, 1969, 23, 2848. 2257. Hariharan, P. C.; Pople, J. A. Theor. Chim. Acta 1973, 28, 213. (25) Domenicano, A. In Stereochemical Applicaiions of Gas-Phase (13) Msller, C.; Plesset, M. S.Phys. Rev. 1934, 46, 618. ; New York, Electron Di/fraciion; Hargittai, I., Hargittai, M., as.VCH: (14) Politzer, P.;Seminario, J. M.; Bolduc, P. R. Chem. Phys. Lett. 1989, 1988; Part B, Chapter 7. 158, 463. (26) Domenicano,A.; Hargittai, I. Accurare Molecular Structures: Their (15) See,e.g.: Candel1,E.; Catalan, J.; Fernandez-Alonso,J. I. Adu. Molec. Determination and Importance; Oxford University Press, 1992.

tigation. Assuming differences from the computed structure between similar parameters has greatly facilitated the structure analysis. The fact that only differences, and not the parameters themselves, have been assumed has largely eliminated the danger of introducing systematic error by these assumptions due to the different physical meaning of the parameters from the two techniques.Z6 3. The structure analyses of 2-nitrophenol and 2-nitroresorcinol have shown the utility of simple resonance schemes in accounting for and even predicting geometrical variations in a series of monosubstituted and ortho-substituted benzene derivatives.