Hydrogen bonding and the structure of substituted ureas - American

and Department of Chemistry, University of Warsaw, 02-093 Warszawa, Pasteara 1, Poland. Received: March 11, 1993; In Final Form: September 10, 1993*...
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J. Phys. Chem. 1993,97,12147-12152

12147

Hydrogen Bonding and the Structure of Substituted Ureas: Solid-state NMR, Vibrational Spectroscopy, and Single-Crystal X-ray Diffraction Studies Waclaw Kolodziejski,+ Iwona Wawer,: Krzysztof Wozniak,: and Jacek Klinowski**+ Department of Chemistry, University of Cambridge, Lensfield Road, Cambridge CB2 I E W,United Kingdom, and Department of Chemistry, University of Warsaw, 02-093 Warszawa, Pasteura I, Poland Received: March 11, 1993;In Final Form: September IO, 1993’

Crystalline N1,Nl-dimethyl-W-arylureas with a para X-substituent (X = C1, Br, and N02) and an ortho hydroxyl group in the phenyl ring (with respect to the urea moiety) were studied by 13CC P / M A S N M R , IR, and Raman spectroscopies and by single-crystal X-ray diffraction (XRD). Details of single-crystal X-ray determination of the structure of N1,N1-dimethyl-W-(2-hydroxy-4-nitrophenyl)urea are given. Solid-state N M R effects related to hydrogen bonding and to molecular conformations were observed. In the NO2 derivative, the carbonyl and hydroxyl groups are in pseudotrans orientation to one another, while in the C1 derivative both pseudocis and pseudotrans orientations occur. For the pseudocis orientation, the hydrogen bond is intramolecular and completes a seven-membered ring, while for the pseudotrans orientation it is intermolecular. The NO2 derivative conformation is largely determined by a transquinoidal resonance between the NO2 group and the urea moiety, mediated by the aromatic ring. The study demonstrates the power of X R D data analysis on the basis of the comparison of experimental and literature results as well as the advantages of using physical methods in tandem for solving complex organic structures.

Introduction

CHART I

Urea is the major end product of nitrogen excretionin mammals, being synthesized by the urea cycle. Urea and its derivatives are used for the production of resins and pharmaceuticals, as a source of nonprotein nitrogen for ruminant livestock, and as fertilizers. Solid urea derivatives are good model compounds for the study of structural effects, because the urea moiety can be differently involved in intra and/or intermolecular hydrogen bonding mediated by N-H proton-donor groups and the lone pairs of electrons on nitrogen and/or oxygen atoms acting as proton acceptors. Hydrogen bonding may thus give rise to a multitude of solid-state structures corresponding to various conformations of substituents at the amide nitrogens within the urea molecules and to different, possibly inequivalent, locations of these molecules in the crystal lattice. We have shown’ that hydrogen bonding substantially affects “hard” structural parameters, such as bond lengths and angles, and that this leads to significant changes in the chemical shift and line splittings in the NMR spectra (the “solid-state NMR effect”).2 Furthermore, we have carried out a liquid-state study of hindered rotation around the C-N(CH3)z bond in N,N-dimethylamides.3 In the solid state such rotation is rather restricted, but we are concerned with the number and geometry of possible conformers. The above arguments justify the choice of N,N1-dimethyl-N2-arylureas(1-111, Chart I) for the present study. The following relevant circumstances must be mentioned. The X-substituent is like to exert a significant structural effect by modifying the electron density distribution in the hydroxyl group and in the urea moiety, thereby affecting hydrogen bonding. Solidstate I3C NMR of aromatic ring carbons should therefore be a very sensitive structure-diagnostic tool, since the -NOz, - O H , and -NHR chemical shift increments are fairly high (cf. the chemical shift additivity scheme for substituted benzenes).4 Furthermore, the I3C methyl signals can be expected to show whether or not the -N(CH3)2 rotation is completely frozen out. The latter question is not trivial and some librations could be inferred for ureas 1-111, since we have observed in solution t University of Cambridge. t University of Warsaw. .Abstract published in Advunce ACS Abstracts, November 1 , 1993.

0022-3654/93/2097-12147$04.00/0

(acetone/CD2Cl2) that at temperatures as low as 163 K rotation is fast enough to produce only one average methyl signal. Organic crystalswith hydrogen-bondednetworkscan be studied by 13Cand lH NMR.S-7 While the former provides information on the molecular backbone, the latter gives more details on the hydrogen bond itself. Since we are mostly concerned with structural elucidation, and since the 1H NMR spectra are not sufficiently well resolved, we present only I3C results obtained with cross-polarization (CP) and magic-angle spinning (MAS). Complementary information is provided by infrared (IR) and Raman spectroscopyand single-crystalX-ray diffraction (XRD).

Experimental Section Infrared spectra were recorded from suspensionsin Nujol using a Zeiss Specord IR-75 spectrometer. Raman spectra were recorded with a Varian Cary 82 spectrometer equipped with a Spectra-Physics argon laser (514.5-nm exciting line, 150-mW power at the sample, spectral band width of the slit 3 cm-1). Single-crystal XRD measurements were carried out using the equipment and conditions listed in Table 111. 13C CP/MAS NMR spectra were recorded at 100.63 MHz using a high-speed double-bearing probehead and zirconia rotors spun in nitrogen gas. Single contact 13C CP/MAS experiments were performed at 298 K with 4-ms contact times for optimal contact time experiments and 50-ps contact times for the shortcontact-time experiments. The length of the IH and l3C u/2 pulses was typically 3 ps, recycle delay 10 s, and MAS rate 8 kHz. The 13C dipolar-dephased (DD) spectra8 were recorded wtih a 50-ps delay prior to acquisition. Dipolar-dephased experiments expose quaternary carbon lines, and the CP experiments with short contact times (SCT)highlight lines from carbons with adjacent protons. Ordinary CP/MAS spectra contain all these lines. We note that ordinary spectra cannot be simply 0 1993 American Chemical Society

12148

Kolodziejski et al.

The Journal of Physical Chemistry, Vol. 97, No. 47, 1993

(a)

100%

(b)

Infrared

Raman 3420

(DMSOi H,O)

I 3600

3200

3500

2400

2800

3400

Wavenumber (cm - ) Figure 1. Vibrational spectra of solids 1-111. 01

H83 1

TABLE I: Comparison of Structural Parameters of Solid 111 with Those of a Hypothetical Mean Structurelb length (A) value (deg) bond

04

Figure 2. Thermal ellipsoid plot of the molecule 111at 50% probability (ORTEP”).

03-N3 04-N3 N3-C4 c445 c443 C5-C6 C3-C2 C641 c2-c 1 c2-02 02-H 1o Cl-N2 N2-Hln N247 C7-N1 C7-01 Nl-C8 Nl-C9

this work average1* 1.217(4) 1.222(4) 1.458(3) 1.383(3) 1.376(3) 1.388(2) 1.387(4) 1.382(2) 1.397(3) 1.360(3) 0.94(8) 1.416(4) 0.92(2) 1.357(3) 1.376(5) 1.229(3) 1.470(4) 1.458(4)

angle

03N304 03N3C4 04N3C4 1.383(3) c 5 c 4 c 3 1.376(3) C4C5C6 1.388(2) c 4 c 3 c 2 1.387(4) C5C6C1 1.382(2) c 3 c 2 c 1 1.397(3) C6ClC2 C6C 1N2 C2ClN2 1.416(4) C l C 2 0 2 0.92(2) C3C202 1.357(3) C202Hlo 1.376(5) C l N 2 H l n 1.229(3) ClN2C7 C7N2Hln N2C701 N2C7N1 01C7N1 C7NlC8 C7NlC9 C8NlC9

this work 123.2(3) 118.4(3) 118.4(3) 121.4(2) 119.5(2) 119.0(2) 120.6(2) 120.7(2) 118.8(2) 125.9(2) 115.3(2) 115.3(2) 124.0(2) 108(2) 114(2) 127.4(2) 118(2) 121.7(2) 116.0(2) 122.3(2) 118.8(2) 124.0(2) 117.1(3)

average12

119.5(5) 120.6(2) 120.8(3) 119.9(2) 119.6(2) 119.6(3) 120.4(6) 120.0(7)

117.2(7) 127.2(3) 115.0(7) 124.3(3) 115.2(3) 120.5(4) 117.1(7) 122.9(10) 118.9(8)

REFCODEs of structures used are the following: BEBLID,BEBLOJ, BEJTEP, BOTRILl 0, BPCTHA, BUFXOP, CDSCBA, CDSCBB, CDSCBC, CDSCBD, CBSCBE, CEJTEQ, CEJTIU, CNBPCT, CPDPSC, CPDPSCl 1, CPFBUR, CPFBUROI, CYCOXB, CYNONB, DPUREA, JAKHEI, KAFCUP, PHUREA, VAPGOI, VEJBOB.

Figure 3. Crystal lattice of solid 111. Thick dashed lines denote intermolecularOH-0 hydrogen bonds, dashed lines weak intramolecular NH-0 hydrogen bonds, and dotted lines very weak intermolecular CH-0 hydrogen bonds.

obtained by the addition of the respective dipolar-dephased and short-contact-time spectra, since in both latter cases the relative line intensities are substantiallydistorted. Conventional I3C NMR spectra of compounds 1-111 in 0.3 M acetone-ds solution were

recorded with a JEOL FX90 spectrometer and were assigned using the chemical shift additivity scheme for substituted benzenes.4 The assignment has been confirmed by W Y H HETCOR and IH COSY experiments (Bruker WP200).

Results and Discussion Vibrational Spectroscopy. Functional groups participating in hydrogen bonding can be monitored by IR and Raman spectroscopy (Figure 1). Thus the N-H stretching band a t 3420 cm-* from solids 1-111 is very sharp in both vibrational spectra and appears at a frequency which is characteristic of free N-H.

The Journal of Physical Chemistry, Vol. 97, No. 47, 1993 12149

H-Bonding and the Structure of Substituted Ureas

TABLE II: Chemical Shifts of Compounds 1-111 in the Solid State (Upper Row) and in Acetone-ds Solution (Lower Row) X N(CH)a C=O c1 c2 c3 c4 c5 C6 CI 35-38 156.5 123.01127.5 147.3 111.6/112.4 126.2 117.2/117.9 120.7 Br NO2

36.6 37 36.4 35.3136.4 36.3

156.6 156.9 156.1 156.0 154.6

126.7 126.5 127.5 133.1 133.2

148.5 147.4 148.7 146.0 145.8

TABLE III: Crystal Data and Details of Data Collection and Refinement of the Structure for Solid 111 general formula C9HiiNn04 fw hir= 225.2 crystal system orthorhombic Pbca space group yellow color 0.30 mm X 0.40 mm X 0.35 mm crystal size a = 7.749(2) A unit cell params

116.9’ 114.1 119.0 107.3 109.4

linear abs coeff electron/unit cell temp diffractometer I9 range for lattice param measurement no. of reflns for lattice param measmt intensity measmts 19 range indices range scan mode intensity control reflns measd every no. of measd reflns criterion for obsd reflns no. of obsd reflns

01 02 03 04 N1 N2 N3 c1 c2 c3 c4 c5 C6 c7 C8 c9

R(U)

correction applied atomic scattering factors solution computer programs used refinement method no. of params refined params refined for non-H atoms for H atoms weighting scheme final R factor RW

graphite monochromatized Cu Ka, X = 1.540 562 A N(CUKa) = 9.38 cm-’ F(OO0) = 944 T = 295 K 20’ 5 I9 530’ 25 0.2-82.5’

0 1 h I 9,O I k l 1 3 , O I I I 26 -29 222,113,131 100 reflns 1877 14 2 4 4 7 ) 1291 0.0 18

Lorentz-polarizationeffects International Tables’ automatic direct methods SHELX76.b SHELXSf full matrix 189

positional and anisotropic thermal params positional and isotropic thermal params unit weights 0.0415 0.04 15 S = 0.85 +0.45, (-0.40) e A-3

goodness-of-fit largest diff peak (hole) in final diff Fourier synth a International Tables for X-ray Crystallography; Kynoch Press: Birmingham,England, 1974;Vol. IV. Sheldrick,G. M.Acra Crystallogr. 1990,A46,467473. Sheldrick,G. M. SHELX76. Program for crystal structure determination. University of Cambridge, England, 1976. Moreover, we have confirmed that the band broadens and practically disappears from the Raman spectrum if the corresponding N-H groups donate protons to hydrogen bonds (cf. I in DMSO/H20). It follows that in solids 1-111 the amide protons are not involved in hydrogen bonding. In all three cases the IR stretching band of 0-H at ca. 3050 cm-1 is very strong, broad, and shifted to low frequency in comparison with the free 0-H band position (3612-3593~m-I).~Thespectra prove that insolids 1-111 hydroxyl protons do take part in hydrogen bonding. Carbonyl bands are not useful in our case because they are not sufficiently specific. A pure C - 0 stretching mode in amides does not exist since it is appreciably coupled with the C-N stretch

119.5 121.2 121.9 118.2 115.6

6,

b = 10.987(2) A = 24.382(4) A V = 2076(1) A3 Z=8 d, = 1.441 gcm-’ d = 1.439 gem-'

KM4 KUMA-diffraction

114.9 141.5 141.5

121.9‘ 118.9 122.5 114.2 117.6

TABLE I V Non-Hydrogen Fractional Atomic Coordinates x 104) and Isotropic/Equivalent Temperature Factors ( X 1V 2) with Esd’s in Parentheses atom xla Ylb TIC

c

unit cell vol molecular multiplicity calcd density measd density radiation

128.1

see text

1766(2) -269(2) 1845(4) 547(4) 1210(3) 1239(3) 1233(4) 1325(3) 483(3) 470(3) 1292(3) 215 l(4) 2179(3) 1424(3) 1356(7) 726(6)

94(1) 3576(1) 424(3) 2141(3) 1625(2) 2003(2) 1344(3) 1768(2) 2630(2) 2495(2) 1491(2) 655(2) 801(2) 1182(2) 791(3) 2880(3)

1671(1) 2637(1) 4483(1) 4557(1) 1079(1) 2006(1) 4284(1) 2567(1) 2906(1) 3459(1) 3690(1) 3370(1) 2811(1) 1584(1) 610(1) 961(1)

46.3(4) 43.7(4) 103(1) 105(1) 44(1) 37.3(4) 65(1) 32.7(4) 33.7(4) 4 ~ 1 ) 43(1) 45(1) 40(1) 36(1) 68(1) 65(1)

Uq is calculated as (U1 l(mtar)(a)2 + U22(b~rar)(b)~ + U33(ntar)(c)2 2(U12(asrar)(bstar))ab cos y + U13(astar)(cstar)ac cos 6 + U23(bstar)(ntar)bccosa))/3,whereustar, bstar,andcstararereciprocal a, b, and c, respectively.

+

TABLE V

Non-Hydrogen Anisotropic Temperature Factors Ed’s in Parentheses

( X 103 A2) with

atom

U11

U22

U33

U12

U13

U23

01 02 03 04 N1 N2 N3 c1 c2 c3 c4 c5 C6 c7 C8 c9

TABLE VI: Hydrogen Fractional Atomic Coordinates ( X 104) and Isotropic Temperature Factors ( X 1V A’) with Ed’s in Parentheses atom xla z/c U Ylb H10 HlN H3 H5 H6 H81 H82 H83 H9 1 H92 H93

-828(41) 837(34) -63(36) 2660(37) 2783(35) 2044(56) 426(76) 1751(57) 1347(44) 756(43) 421(67)

4073(28) 2760(25) 3012(25) -9(25) 253(23) 1183(39) 915(54) 112(42) 3421(31) 3052(30) 3038(47)

2898(12) 1920(10) 3694(11) 3547(10) 2577(10) 335(17) 376(23) 696(17) 1191(14) 588(14) 1168(20)

77( 10) 55W 58W 59(8) 53(8) 128(16) 199(28) 135(19) 85(11) 88( 11) 170(22)

and to some extent with the N-H bend? The situation becomes more complicated if, as four ureas 1-111, there are two inequivalent C-N bonds and the bonds of interest are conjugated with the aromatic ring. In arylureas the carbonyl function contributes to two bands,lO which in our spectra appear at 1640 and 1588-1 595 cm-1 (not shown). The two bands cannot be specifically assigned

Kolodziejski et al.

12150 The Journal of Physical Chemistry, Vol. 97,No. 47, 1993 43:

MI

35.3

Short CP contact time

c2

A

C=O

Sample I1

I

? dipolar dephased

,

117.9

1473

spectrum

160

40.0

37.5

35.0

32.5

ppm from TMS

Figure 4. 13C MAS NMR lines from the methyl groups of solids 1-111.

to hydrogen-bonded and free carbonyls, since both are present for solid 111, where all carbonyls participate in hydrogen bonding (see the XRD results). It appears that the pair of bands at 1640 and ca. 1590 cm-1 is probably an intrinsic feature of the arylurea grouping.9J0 Interpretation is further obscured by the fact that in this spectral region two bands from the aromatic ring also occur.9 X-ray Diffraction. We present XRD data only for urea 111, because it has not been possible to obtain satisfactory crystals of the others. The structure has been solved with high accuracy (cf. Tables 111-VI), so fortunately there is more information just on this compound for which the Raman results are lacking (the colored sample is damaged by the laser beam). We have found that molecules of III (Figure 2) are structurally equivalent in the crystal lattice and form intermolecular hydrogen bonds involving hydroxyl and carbonyl groups (Figure 3). The molecules are fairly planar, which can be deduced from the angles involving the planes of the aromatic ring (a),the urea moiety (n),the carbons bound to N1 (Z),and the NO2 group (r).The angles an, OZ, M', nZ,nr, and ZI' are 21.6, 20.7, 5.0, 1.7, 17.3, and 16.6', respectively. Furthermore, the urea moiety is more nearly planar than reported in the literature.12 Thus C7 is displaced from the N201N1 plane by 0.001 A (average value12 0.005 A), and N1 is displaced from the C7C8C9 plane by 0.022 A (average valueI2 0.038 A). We note that in compound111theN2C7 bond is longer by 0.039 A than the C7N1 bond (Table I), while in the mean structure it is shorter by 0.018 A.12 The intermolecular hydrogen bond is almost linear since the 0 1 H 0 2 angle is 174(2)O. The distance between carbonyl (01)

140

120

100

ppm from TMS

Figure 5. Aromatic region of the '3C MAS NMR spectrum of solid I.

and hydroxyl (02) oxygens is 2.641(3) A, and the 01--H and H-.02 distances are 1.70(3) and 0.94(8) A, respectively. A short 02-Hln contact [2.14(3) A] as well as the angle C l N 2 H l n [ 114(2)O] suggests the presence of a weak intramolecular hydrogen bond between 0 2 and Hln-N2 [02-*Hln-N2 = 109(2)OI. There is also a quite short intermolecular contact of 2.53(3) A between H92 of CH, antiperiplanar to C = O and oxygen 0 4 of NO2 (Figure 3). We describe this contact as "quite short" because it is shorter than most CH-0 contacts in quinone derivatives,where such interactions are responsible for the threedimensional arrangement of molecules. This very weak intermolecular CH-0 hydrogen-bond-type interaction binds the molecules in the Z dimension (Figure 3). Solid-stateNMR. Solid-stateNMR signalshave been assigned on the basis of chemical shifts in solution as well as DD and SCT experiments (Table 11). Methyl and carbonyl signals are easily identifiedby their chemical shifts. We note that the methyl signal of I (Figure 4) is a superposition of four components and that of 111 contains two components. The methyl signal of I1 is broad and structureless. Assignment of the aromatic spectral region (Table 11 and Figures 5-7) is not straightforward and has been performed as follows. Quaternary C2 signals at 146-147 ppm appear with a similar chemical shift in both the liquid and solid states, Aromatic signals with the lowest chemical shift correspond to C3. In order to justify the latter assignment for solid 11, we argue that the signal at 114.1 ppm appears in the SCT spectrum and is absent in the DD spectrum. Thus the signal comes from a hydrogenbearing carbon and cannot be assigned to C4 as suggested by the liquid-state chemical shift.

H-Bonding and the Structure of Substituted Ureas

The Journal of Physical Chemistry, Vol. 97,No. 47, 1993 12151

Short CP contact time Short CP contact time

c3

h

V

c2

c4 I

c '0

dipolar dephased

ordinary spectrum 156.9

147.4

121.2

160

140

120

100

ppm lrom TMS

Figure 7. Aromatic region of the I3CMAS NMR spectrum of solid 111. 160

140

120

100

ppm from TMS

Figure 6. Aromatic region of the 13C MAS NMR spectrum of solid 11. Asterisks denote a signal of C4 split by coupling with 79Brand *lBr (both I = 31~).

The C1 signal of I1 and the C1 and C4 signals of 111 can be readily assigned by considering their chemical shifts and their presence in the DD spectra and absence in the SCT spectra. The C4 signal of I1 is very broad and split by a C4-Br interaction, but it can be recognized in the DD spectrum. We suppose that the interaction involves the large quadrupole moments of 79Br and *lBrand is modified by MAS and in the 79Br-13Ccase maybe also by ~elf-decoupling,~~ caused by their similar resonance frequencies. At this stagewe are not able to explain these features of the signals. In any case, the effect is irrelevant to the problems under consideration. Carbons C1 and C4 of I have close chemical shifts in solution. In the solid state we find two signals in this spectral range: a doublet at 127.5/123.0 ppm and a singlet at 126.2 ppm (both present in the DD spectrum and absent in the SCT spectrum). We recall that for I there are four methyl signals and that the carbonyl group can be in pseudocis (cf. Chart I) or pseudotrans (cf. Figure 1) conformations with respect to the hydroxyl group. Each of the orientations corresponds to two inequivalent methyl groups, so that if both carbonyl orientations occurred, one should expect only four methyl signals. This implies that the C1 signal of I could be split into a doublet corresponding to the pseudocis and pseudotranscarbonylorientations. It follows that thedoublet at 127.5/123.0 ppm should be assigned to C1 and the signal at 126.2 ppm to C4. A smaller splitting observed for C3 (11 1.6/ 112.4 ppm, according to our previous assignment) reflects its remoteness from the perturbation. We note that there is another

signal from I (at 117.2/ 117.9 ppm), present in the SCT spectrum and absent from the DD spectrum, with splitting similar to that observed for C3. This must be assigned to another meta carbon with respect to C1, that is, to C5. The final signal to be assigned (at 120.7 ppm), present in the SCT spectrum and absent from the DD spectrum, comes from the only carbon which has not been considered for I, that is, from C6. We note that the C3 signal of I1 is broad because C3 is located close to Br. The same should be observed for C5, so the broad signal at 118.9 ppm corresponds to the latter carbon, and the remaining sharper signal at 121.2 ppm to C6. The signals from C3 and C5 show similar behavior (the splittings for I and the line width effects for II), which may assist us in assigning the C5 and C6 signals of 111. The C3 signal of solid I11 has the chemical shift lower than that in solution, so we ascribe the signal at 114.2 ppm to C5 and that at 115.6 ppm to C6. The latter assignment is consistent with the order of the C6, C5, and C3 signals in the spectra of I and 11. However, since it relies on an analogy, it must be regarded as tentative.

Conclusions Solid-state NMR results for compound 111are consistent with its IR spectrum and with the structure determined by singlecrystal XRD, in which the carbonyl and hydroxyl groups are in mutual pseudotrans orientations (Figure 1) and form intermolecular hydrogen bonds (Figure 2). NMR shows that for compound I both pseudocis (Chart I) and pseudotrans orientations are present. Since all hydroxyl protons participate in hydrogen bonding but amide protons are not involved (IR and Raman results), we suggest that the hydrogen bond links carbonyl and hydroxyl oxygens in both conformations. For the pseudocis orientation, the hydrogen bond is intramolecular and

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12152 The Journal of Physical Chemistry, Vol. 97, No. 47, 1993

SCHEME I

B

t

D

C

A

E completes a seven-membered ring (3C, 20, l N , and 1H)while for the pseudotrans orientation it has intermolecular character. A cis structure with a O-H.-N hydrogen bond between the hydroxyl and amine groups can be ruled out as too strained. We do not know yet whether both pseudocis and pseudotrans orientations in I occur in the same crystal latticeor in the different polymorphs. The crystal structure of solid I1 has not been solved. Since the C1, C3, and C5 I3C MAS NMR signals are not split (Table 11, Figures 5-7), molecules of IIcould exist only in one conformation, i.e. the pseudocis or pseudotrans. The methyl signal seems to be a composite (Figure 4), but being broad and unresolved, it is of little value for spectral assignment. Since vibrational spectra of I1 are similar to those of I and 111, it is likely that hydrogen bonds involving hydroxyl and carbonyl groups are also formed in this case. Structural differences between solidsI and 111can be explained using Scheme I, which gives five of the many mesomeric structures possible for compound 111. Structures A, C, and E account for the partly double character of C-N amide bondsI4 within the urea moiety. Structures D and E describe a transquinoidal resonance between the electron-accepting (-M)I4 nitro group and the electron-donating (+M)14 urea moiety, mediated by the aromatic ring. Structures B-E show that lone electron pairs on nitrogens participate in a resonance either with the carbonyl bond or with the nitro group and are therefore reluctant to be engaged in hydrogen bonding. A contribution from the structure Eshould make bonds C5-C6, C3-C2, Cl-N2, and C7-N1 shorter, while bonds C4-C5, C W 3 , C6-C1, C2-C1, N2-C7, and C7-01 should become longer. This is really observed if one compares data for 111with those for the hypothetical mean structure12given in Table I. The mean structure was calculated by averaging bond lengths and anglesof 28 residuals of arylureas with hydrogen and aryl N2 and different substituents in the aromatic ring and at N1. Structural parameters of these residuals were taken from all 26 relevant structures found in the Cambridge Structure Database. The only exception is theC4-CS bond, where an effect of the nitro group on u electrons can be involved. We conclude

that the mesomeric structure E is of considerable importance to molecule 111and, since it contains a system of conjugated double bonds, the molecule is fairly flat. This may prevent the molecule in the pseudocis conformation from adjusting its geometry to form the intramolecular O-H-.O=C hydrogen bond. On the other hand, for the electron-donor substituent, such as chlorine (+M),l4 only mesomeric structures like A-C are possible, so the resonance effects between the aromatic ring and the urea moiety are less important. It follows that the molecule of I is less planar and more flexible and is capable of adapting to both hydrogenbonded conformations, pseudocis and pseudotrans.

Acknowledgment. We are grateful to Unilever Research, Port Sunlight, for support, to Dr. Vera Koleva (Sofia University, Bulgaria) for supplying the ureas, and to Professors Zbigniew Kecki and Tadeusz M. Krygowski for discussions. References and Notes (1) Wozniak, K.; Krygowski, T. M.; Grech, E.; Kolodziejski, W.; Klinowski, J. J. Phys. Chem. 1993, 97, 1862. (2) Fyfe, C. A. SolidState NMRfor Chemists;CRC Press: Boca Raton, FL, 1984. (3) Wawer, I.; Kolodziejski,W.Ber. Bunsen-Ges.Phys. Chem. 1988,92, 637. (4) Wehrli, F. W.; Wirthlin, T. Interpretation of Carbon-I3 NMR Spectra; Heyden: London, 1976. ( 5 ) Etter. M. C. LecturenotesonStatic. KinrmaticandDv~micAsaccts of Cijstal anh Molecular Structure; 18th Course International Schdol of Crystallography, Erice; p 123. (6) Etter. M. C.; Voita, R. L. J. Mol. Graphics 1989, 7 , 3. (7) Etter, M. C.; Rcntzel, S.M.;Voita, R. L. J. Mol. Srwct. 1990,237,

."*. 165

(8) Opella, S. J.; Frey. M. H. J. J. Am. Chem. Soc. 1979, 101, 5854.

(9) Bellamy. L. J. The Infrared Spectra of Complex Molecules; 3rd 4.; Chapman and Hall: London, 1975; Vol. 1. (IO) Kutepov, D. F.; Dubov, S . S.Zh. Obshch. Khim. 1960, 30, 3448. (1 1) Johnson, C. K. Report ORNL-5138; Oak Ridge National Laboratory: Oak Ridge, TN, 1976. (12) Allen, F. H.;Davies, J. E.;Galloy, J. J.; Johnson, 0.; Kennard, 0.; Macrae, C. F.;Mitchell, E. M.;Mitchell, G. F.;Smith, J. M.; Watson, D. G. J. Chem. Id.Comput. Sei. 1991, 31, 187. (13) Levy, G. C. J. Chem. Soc., Chem. Commun. 1972, 352. (14) March, J. Advanced Organic Chemistry: Reactions, Mechanisms and Structure, 2nd 4.;McGraw-Hill: Kogakusha. Tokyo, 1977.