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Molecular structure and ring distortions of p-diiso-cyanobenzene in the gaseous phase and in the crystal
Joumal of hfoleeuti Simcture, 125 (1984) 19-32 Elsevier Science Publishers B-V., Amsterdam-Printed
in The Netherlands
MOLECULAR STRUCTURE AND RING DISTORTIONS OF p-DIISCb CYANOBENZENE IN TEE GASEOUS PHASE AND IN T-HE CRYSTAL*
MARCELLO
coL&AmETRo
Institute of Stmctural Chemistry. ALDO
DOMENICANO**.
CNR,
GUSTAV0
I-00016
Monterotondo
PORTALGNE
and lNBS
Department of Chemisby and Institute of P?zarnuzccuticul Rome ‘La Sopienza ‘, I-00185 Rome (Italy) ISPVbLN
TGRRINI
Chemisfry.
Uniuersity
of
of Materials
Science.
The
Uniwrsity
SCHULTZ
Hungarivr Academy of Sciences. Research L-aboratov ment of Structural Studieq pf 1 I 7. H-l 43 1 Budarzt (Received
(Italy)
HARGITPAI**~*f*
Departments of Chemistry and Physics and InsBtute of Connecticuf Storrs. CT 06268 (U.S.A.) GYGRGY
Stazione
8 May
for Inoganic (Hung-)
Chemism.
Depcut-
1984)
The molecular structure of p-diisocyanobenzenr has been accurately determined in the gaseous phase (by electron diffraction) and in the crystal (by X-ray aystallography). The X-ray diffmction results have been corrected for the effects of solid state thermal motions and asphericities in valence eleclrons, so as to allow a comparison with gasphase rsulk In both phw the benzene ring is found to deviate appreciably from D, h symmetry: the most pronounced distortion is an increase to - 122” of the internal angle at the ipso carbon, e The geometry of the sulxtituent indicates that the dipolar form -fi& contributes to the electronic structure substantially more than does -N=C_ The following structural change occur on going from the free molecule to the crysta+l molecule: (i) an increase of Q from 121.7 2 0.2” to 122_2(3)“,t(ii) an increase of r(C-N) from 1.388 2 0.003 A to l-396(3) A, and (iii) a decrease of r(N&) from 1.176 = O-002 A to l-166(4) A. The consistent pattern of these changes indicates that the contribution of the dipolar form of the substituent is larger in the crystalhne than in the gaseous state. The crystals are composed of stacks of parallel molecules arranged in such a way that each benzene ring is sandwiched between two antiparallel isocyano groups, at a distance of 3.45 _&_ This arrangement appears to stabilize the dipolar form of the substituent. which has a v* orbital properly oriented to favour a charge-transfer interaction with a P orbital of the neighbouring benzene ring. *A preliminary report of this work has been presented elsewhere [ 1 I**To whom all correspondence should be addressed **+Viiting professor, on leave from the Hungarian Academy of Sciences.
0022-2860/84/$03.00
g) 1984
Elsevier
Science
Publishers
B-V-
Budapest
30
The effects of the crystal field on molecular struct-uze can best be ascertained by accurately determining the geometry of the same molecule in the gaseous and solid statis. We have recently studied the molecular structure of p-dicyanobenzene in the two states, by gas electron diffraction and X-ray crystallography, respectively [2]
_ THE ELECTRON DIFFRACTION STUDY
Experimental
and &ta
reduction
p-Diisocyanobenzene was prepared according to ref. 5, and purified by vacuum sublimation followed by recrystallization from ethyl ether/n-hexane. The electron diffraction patterns were taken in Budapest with a modified EG-100A apparatus [6], using the so-called membrane nozzle system [7] _ The nozzle temperature was about 111 “C!; nozzle-to-plate distances of about 50 and 19 cm were used. The treatment of the experimental data was the same as in ref. 8. The ranges of the intensity data used in the analysis were 2.900 < s S 14.000 A-’ and 9.25 < s < 36.00 A-‘, with data intervals As = 0.125 and 0.25 A-‘, respectively_ The total experimental intensities are available elsewhere [from the British Library Lending Division, Wetherby, Yorks., as Supplementary Publication No. SUP 26270 (11 pages)] _ The fmal molecular intensities are shown in Fig. l_ The esperimenti and theoretical radial distributions are presented in Fig 2_ They were calculated us&g au artificial damping factor equal to 0.002 A’. Theoretical values vere used in the 0.00 < SC 2_00A-’ region. The radial distribution closely resembles that obtained for p+Scyanobc?nzene [2] _ Structure analysis The least-squares method was applied to the mo!ecular intensities in the same manner as in refs. 2 and 8, assuming D = molecular symmew_ The best fit to the experimental data (R = 0.0321) was given by the molecular parameters presented in Table 1. Twenty-five independent parameters were varied in this refmement; among these were six geometrical parameters, five shrinkzg?s a.nd nine uncoupled amplitudes of vibration. Nine other amplitudes were coupled in three blocks and refined under the constraint of constant differences within each block (see Table 1). *An early two-dimensional study pletely inadequate for the purpoe
of the crystal structure of this compound of the present investigation
[3]
was com-
21
P-
-0
5
10
-24.. 15
c&(Nc)2
--^ 20
25
s-A-e+
30
5.h’
35
Fig_ 1_ Molecular intensity curves for the two camera ranges [(- - -) experimental; theoretical]. Also shown are the difference curves (experimental - tkoreticall
(-_)
Fig_ 2. Radial distribution CUI-VCS I(---) experimental; (-) theoretical]. The position of the most important distances is marked with vertical bars, the height of which is pmportional to the weight of the distances Also shown is the difference curve (experimental theoretical).
22
TABLE
1
Final molecular param r& (a) Distances
km,
and mewz amplibtdes
the electron diffrstior of vibration
study
(A)
Atomic P&
Multiplicity
‘*
I
Key to the coupling szbeme
Cl-C2 Cl-N7 N7-C9 c-H2
6 2 2 4 4
Cl - -. c4 C2---C5 c2- - - C6 N7---C2 N7---C3 N7---C4 NF---NS c9---Cl c9---c2 c9---c3 c9---c4 C9---N8 c9--- Cl0 Cl ---H2 Cl---H3 C2---H3 C2---H5 C2---H6 N7---H2 N7 - - - H3 C9---H2 C9---H3 H2---H3 H2---H5 H2---H6
1 2 2 4 4 2 1 2 4 4 2 2 1 4 4 4 4 4 4 4 4 4 2 2 2
O-0512(4) 0.0562 0.040(l) 0.077= 0.059(l) O-058(3) 0.073 O-059 0.067 O-069(2) O-075(3) O-081(9) O-057 0.110(3) O-097(3) O-071(5) 0_087(6) O.OSS(ZOj 0.100= 0.100= 0_100= 0.100= 0.100= 0 125= 0:125c 0.135= o-135= 0.140= 0.140= 0_140=
i i ii
Cl---C3
l-3980(4) 1.386(lP 1.1747(5) 1.083(3) 2.411(1)5 2_759(3F 2.814(2p 2442(2p 2401(l)b 3.674(l)b 4.145(2)b 5_531(3)b 2_561( l)b 3.456(4)b 4_773(4)b 5_304(5)b 6.675(7)b 7_852(23p 2.161(9)b 3.396(7)b 2157(10)b 3_897(4)b 3.421(7)b 2.639(15)b 4_553(lO)b 3_452(16)b 5.615(11)b 2489(29)b 4.979(7)b 4_313(17)b
(b) Angkr
c). dmerences
~cHl-C6 LC~-C~-H% A (C-NC)e 6(C9---C2) 6(C9---C3) &(C9---C4) 6(C9---NS) 6(C9---ClO)
between
bond
distances
(A).
Vake
Parameter (a)
and shrink.agesd
___ ul
iv
iv .._ ll1 .._
111 v
vi vii --_ 1u ___
VIII ix X
xi xii
(A)
121.74(11) 120.2(9) +J.O12(2) 0.008(4) 0. @25(4) O-017(6) 0.031(8) O-029(23)
‘Least-squares standard deviations are given in parentheses as units k the last digit bDependent parameter. cAsu.medd. dThe shrinkage is defined here as the decrease in a non-bonded distance when refmed as an independent parameter, as compared with the due calculated by employing geometrical constcaints. 'A (C-NC)= r(Cl-N7)-r(Cl-C2).
23
Many altemative values were tested for the differences between the amplitudes of vibration coupled in each block. The geometrical parameters of the heavy-atom skeleton were found to be virtuaUy insensitive to the choice of these differences, witi the noticeable exception of the parameter A(C-C) = r(Cl-C2) - r(C2-C3)_ Thus only a mean value of r(C-C) was eventually refined_* The value assumed for AZ = Z(C9 - - - Cl) - Z(C1 -- - C3) was found to affect appreciably some of the shrinkages and the C3<2-II2 angle. The shrinkages S(C9---C2), S(C9---C3), S(C9---C4), s(C9---N8) and 6(C9---ClO) were all successfully refined as independent parameters. 6(C9--C2) and 6(C9--C4) appeared to be somewhat sensitive to the conditions of the analysis, but in no case were they found to vanish or to lxcome negative. Refinement of shrinkages for the C9- - -Cl and N7 - - - N& distances was attempted, but this led to very small or negative values. Ignoring the shrinkage effect seriously worsened the fit (R = 0.0378) and caused the angles Q and C3-C2-H2 to increase by 0.4” and l-2’, respectively_ THE X-RAY DIFFRACTION STUDY Experimen
taaland data reduction
Colourless crystals suitable for X-ray diffraction work were grown by vacuum sublimation_ They were found to decompose rapidly in the X-ray beam unless sealed in 8 glass capillary. Accurate unit cell parameters were dete-mined by leastsquares techniques from the e-values of 37 reflexions, e-range i7-22” (MO KQ radiation), centred on the Syntex P2, difc%actometer of the Institute of Stxuctural Chemistry (Monterotondo Stazione). The content of the monoclinic unit cell is two molecules of pdiisocyanobenzene. Crystal data are reported in Table 2_ TABLE
2
Crystal dataa: pdiisocyanobenzene, C,H,N,, dec.). Monoclinic, space group A2/m, 2 = 2. a b C
*The asumptior; product
F.W.
4.772( 3) A 7.232(3) A 9.887(5) A of a(C-C)
= +O.Oll
128-14,
mp. 165-166-C
D, as from
the X-ray
K,
98_2a(5)” 337.7( 3) A” 1.260 g cm-
8 V
A,
(438-439
diffraction
study,
had only
A, as from tbe X-ray diffraction *The assumption of A(C<) = rO.O1l marginal effects on the I? factir and on the other geometrical parameter
study,
had oniy
24
Two different crystals were used for data collection. A rather large crystal, O-38 X O-63 X 0.70 mm3, was used to measure most of the intensities_ A smaller crystal (0.30 X O-35 X O-18 nun3) was used to obtain accurate intensities in the low-0 region. The intensity data were collected at room temperature (295-300 K) in the e-20 scan mode with graphite-monochromatized MO KQ radiation_ The scanning intenml was from 28 (KQ,) - 1.5” to 28 (Kc!*) + 1.7” for the larger crystal, and i?orn !i?d(K~,)- 1.2” to 28(Ka,) + 1.6’ for the smaller. The scanning rate was varied according to intensity, fiorn a minimum of 0.017” s-’ to a maximum of 0.49” s-l. The background was measured for l/6 of the scanning time at each end of the scan. The reciprocal space was explored from 0 = 1.5” to 35” for the larger crystal, and from 19 = 1.5” to 25” for the smaller one. Four standard reflexions monitored periodicaly did not indicate crystal decomposition. The intensities were corrected for Lorentz and polarization effects, but not for absorption [r(Mo -Ku) is 0.086 mm-’ ] _ For the larger c_rystal the reflexions measured were 1604, of which 834, having I > 3.5 cr. were considered as non-zero_ Both the hkl and i&l reflexions were measured. Averaging and merging [internal R(F,) = 0.0491 led to a set of 412 independent observations. For the smaller crystal 298 hkl and hxI reCexi0n.s with I > 8 or were used. The intensity data from the two crystals were put on the same scale using common reflexions [internal R(Fo) = 0.062] and merged into a single data set of 415 independent observations. Structure
determination
and refinement
The observed systematic extinctions (hkl, k + 1 = 2n + 1) are consistent with the space groups A 2, Am, A2/m. The Patterson synthesis led to an unambiguous solution in A2im_ ‘Ibe two molecules contained in the unit cell have site symmetry 2/m (C,); the mirror is perpendicular to -he benzene ring and contains the long axis of the molecule_ The atomic parameters produced by the Patterson synthesis were refined by least-squares techniques. -The hydrogen atom was located in a difference synthesis_ The final refinement was by full-ma& least-squares techniques, minimizing Cw(lFO I - klF,I)‘, with the non-hydrogen aroms treated anisotropically and the hydrogen atom isotropically_ Weights were given according to the empirical function w = l/(a f blFo I I clFO I’) 19, lo], with u = 4-5, b = 1-O and c = 0.44_ The atomic form factors were the same as in = ref. 4b. The number of observations per refined p ammeter was 415j32 13-O_ The final R and R, were 0.0333 and 0.0543, respectively, with all shifts less than 0.02 0. The final atomic coordinates are given in Table 3. Observed and calculated structure factors and anisotropic thermal parameters are available as supplementary material. A ‘high4 refinement’ was also carried out, using only reflexions with sin 19/x > 0.55 A-’ (0 > 2?), and treating the hydrogen atoms as in ref. 2.
25 TABLE Final
3 atomic
-rdinateP
la) Non-hydrogen
atomsb
Atom
‘,0.X
Cl
8314(3) 8304(5) 9149(2) 9139(5) 6609(a) 6614(S)
c2 N7
c9 (b) Hydrogen Atom H2
from
5193(4) 5168(i) atomd 10% 853(3)
the X-ray
diffraction
10.y
study
104
B c,c(-q=)
1005(2) 1004(2) 507(l) 5(39(3) 2044(1 j 204 l(2)
3.35 3.13 3-81 3.52 3.91 3.68
2699( 2) 2593(3)
4.86 4-50
1 O’y
10’2
B (a’)
280(2)
69(l)
28(3)
d
0 1676(l) 1680(l) 0 0 0 0
aLeast-s.quares standard deviations are given in parentheas units in the last digit_ The coordinates of C6 and H6 are obtained from those of C2 aud HZ respectively, by applying the symmetry operation x, y, z_ The coordinates of C3, C4, C5, H3. H5, N8 and Cl0 are obtained from those of C6. Cl, C2. H6, H2, N7 and C9, respectively. by applying the symmetry operation 2 -z, y. ---2. bFor each atom the values on the top line refer to the conventional refinement, those on the bottom line to the high-e refinement_ =Defiied as (B,B;B,j’“. where Bi = 8n’q (i = 1, 2. 3)_ The tYi are the r_m.r amplitudes of vibration along the principal axes of the thermal ellipsoid aFrom the conventional reflnemeut
Compared with the ‘conventional’ refinement based on aJl reflexions this refiiement gives atomic positions which are closer to nuclear posit-ions. The number of observations per refined parameter was 206/28 = 7.4; unit weights were given to all reflexions_ The final R and IL, were 0.0319 and 0.0363, respectively, with all shifts less than 0.01 u. The resulting atomic coordinates are presented in Table 3; anisotropic thermal parameters are available as supplementary material. Solid-state
thermal motions
The motion of the molecule in the crystal shows a clear anisotropy, with the major axes of all thermal ellipsoids roughly perpendicular to the molecular plane (Fig. 3)_ This s=sgests that the contribution corn internal vibrations is smaller than that tirn translational and librational rigid-body motions. We have thus carried out a thermal motion analysis in terms of rigid-body motions, using Schomaker and Trueblood’s approach [ll] _ The elements of the translational (T) and librational (L) tensors have been derived by least-squares techniques from the anisotiopic thermal paxamet-en U, (the eIements of the mean-square displacement tensor of each
26
Fig 3. An ORTEP drawing of thep-diisocyanobenzene molecule, showing the anisotropy of thermal motion in the crystal_ The thermal ellipsoids of the non-hydrogen atoms have been scaled to s 50% probability IeveL (a) Projection onto the plane of the benzene ring. (b) Projection onto the crystallographic mirror (hydrogen atoms have been omitted for clarity)_ Tbe drawing is based on the atomic p arameters from the high+ refinement
atom) of the high8 refinement. The agreement between observed and calculated U, values is good: R, = ~lAU,l/~IU,I = 0.0251.The r.m.s. values of the differences be~;ueen observed and calculated Uji’s, and of the estimated standard deviations aUti, are <@UJ’ > ‘I* = 0.00095 A’ and <(DU# > 11* = 0.00068 A*, respectively. The elements of the T and L tensors are available as supplementary material. Their eigenvalues and eigenvectors (which give the magnitudes and directions of the principal components) are given in Table 4 together with thou of the inertial tensor I. The librational motion is seen to be highly anisotropic, with the principal axes of L closely corresponding to those of I. As in p-dicyano’benzece [2], the largest oscillation
27 TABLE Analysis
4 of solid-state
(a) Eigerwalues
Tepsor T
L
I
thermal
motiona
and eigenvectom of the irans1atior.a~ Eigenvalues
T,
0.040
T, T, L, Lz L, 1,
O-036 0.035 0.0169 radl 0.0027 O-0015 722.4 anxu A=
1, I,
651.5 70.9
1ibmtionaI and inertiaIb tensors EigenvectorS 0.12
Al
0.00 0.99 O-76 0.00 O-65 0.72 0.06 -0-69
0.99 0.00 -0.12 -0.65 0.00 O-76 0.69 0.00 0.72
0.00 -1.00 0.00 O-00 1.00 0.00 o-00 1.00 0.00
(b) R.mr amplitudes of vibration along the principal axes of the translational and libmtional tensors Tensor Amplitudes (A) Tensor Amplitudes 0 T
O-20 0.19 0.19
L
i.4 3.0 22
aBased on the anisotropic thermal parameters from the high-e refinement. bHeavy-atom framework. The orthononnal reference system used in the analysis (a’, b’. c’ ) has the a’ and b’ apes coinciding with the a and b axes, respectively, of the monoclinic system (4
b, cl-
with the fact that no shrinkage of the Cl - - - C9 distance was observed in the electron diffraction study. It appears that the isocyano group in the present molecule is much more rigid than the cyvlo group in p-dicyanobenzene [2] _ Calculations for the X-ray and electron diffraction studies were carried out on the Univac 1100/82 computer of the University of Rome and on the HP 21MX minicomputer of the CNR Research Area (Monterotondo Stazione), using the same programs as in ref. 2. RESULTS
AND
DISCUSSION
The free molecule
The gas-phase structure of p-diisocyanobenzene, as obtained fi-om the electron diffraction study, is reported in Fig. 4d. The benzene ring is seen to deviate significantly from Ddh symmetry. The rather high value of the endocyclic angle at the ipso atom, a = 121.7 2 O-2”, is a clear indication of the strong u-electron withdrawing character of the isocyano group [14] _ In gaseous p-dicyanobenzene [2] the corresponding angle was found to be slightly, though perhaps not significantly, larger, a = 122.1 f 0.29 As the
28
Fig. 4. Molecular geometry of p-diisocyanobenzene, as determined by (a) X-ray crystallography, conventional refinement; (b) X-ray crystallography, high-0 refiiement; (c) as (b), after correction for libration; and (d) gas electron diffraction Bond distances are given in A, angk in degrees; the distance in (d) are rs valuex Only an average value of the ring C-C bond distances has been determined by elech-on diffraction. Here and throughout this paper total errors (estimated as in ref. 13) are given as error limits; leastsquaresstandard deviations are given in parenthesg as units in the last digit
group is generally considered to be more polar than the cyan0 a diEerence with the opposite sign might have been expected. gfonp*, The mean length of the ring C-C bonds in p-diisocyanobenzene is = 1.400 + 0.003 A, compared with 1.397 f 0.003 A in pr’g(C-c),, dicyanobenzene [2] _ Although the difference is not significant, the slightly larger value obtained in the present molecule is consistent with the smailer value of Q [lS] _ The N7-C9 bond length, rg = l-176 k O-002 A, is in the range expected for a triple bond, thus indicating that the canonical form (I) is the main contributor to the structure of the molecule_ Nevertheless, comparison with p-dicyanobenzene, r,(CEN) = 1-167 + 0.002 A [2], suggests that COPtzibutions from canonical forms like (II) and (III) may also be of importance.
isocyano
*The values of Taft’s inductive param et=. up are 0.67 for the isocyano group and 0.57 for the cyano group [ 15]_ It should be recognired, however. that these values originate from
solution studies (they have been derived from the pK% of and thus may not be transferable to the free molecules_
in water).
substituted acetic acids
29
(II
(D3
(IlIt1
The structure of the monosubstituted derivative, isocyanobenzene (phenylisocyanide) has been investig+l by microwave spectroscopy [17] _ Possible bond distances for the C-N=C lkagment have been derived under various alternative assumptions from the spectra of the parent compound and l+ee deuterated species. The valu+,es_obtained fall in rather wide ranges, = l-400-1.416 A and r,(N=C) = 1.136-1.160 A. The correro(C-N) sponding rs values from the present study are considerably different, 1.388 f 0.003 and 1.176 f 0.002 A, respectively. Needless to say, the molecular structure of isocyanobenzene calls for further examination. The crystal
structure
As with the cyan0 isomer, the crystals of pdiisocyanobenzene are composed of layers of parallel molecules. The molecular plane is close to the i02 plane, which gives by far the most intense reflexion of the diffraction pattern. The relative positions of the molecules in the two isomers are completely different, however_ In p-dicyanob enzene the benzene rings are partially overlapped, and the cyan0 groups take part in a complex system of intermolecular interactions that involve primarily antiparallel groups from adjacent molecules 12, lS]_ In p-diisocyanobenzene each benzene ring is sandwiched between two antiparallel isocyano groups, at a distance of 3-45 A (Fig 5). As in many molecular complexes of p-benzoquinone [19], this arrangement is favourable to a charg&ranxfer interaction, involving a 1~orbital of the benzene ring and a v * orbital of the adjacent isocyano group. The actual transfer of charge may be quite low, or even absent, however, as in the pyren~tctramethyluric acid 1:l complex [20] or in the ‘self complex’ of 1,4_naphthoquinone [21]. It is possible that the peculiar arrangement of planar molecules in these systems is required to maximize the contribution of the highly polar groups to the dispersion forces [19] _ The complete environment of the isocyano group in the crystals of pdiisocyanobenzene is shown in Fig. 6. The role of antiparallel interactions between adjacent functional groups is seen to be much less important here than inpdicyanobenzene [2] _
Fig 5. Schematic projection of the crystal structure of p-diisocyanobenzene The mclecules at O&Jhave been omittd for clarity.
down b_
The crystal .nolecule The molecular geometries of p-diisocyanobenzene obtained from the X-ray diffraction study are reported in Fig. 4a-c. The geometry most directly comparable with that fiorn the electron diffraction study is that of Fig. 4c, where the atomic cm&mates from the high4 refinement have been corrected for ‘Jle effects of libration. The correction is particularly important in the present case, as the crystal molecule undergoes a high-amplitude ‘rolling’ about the C9 - - - Cl0 axis. The site symmetry of the crystal molecule is Czh, but the benzene ring has Dlh symmetry, within the limits of experimental error (Table 5). The substituent is slightly bent out of the plane of the ring. A consistent pattern of minute structural changes occurs as the molecule of p-dikocyanobenzzne is frozen in the crystal. These include: (i) a 0.5” increase in the a angle, (ii) a 0.008 A increase of r(Cl-N7). and (iii) a 0.010 ZL decrease of r(N7-C9) _* Though small, these changes are beyond experimental error. When taken together, they indicate th_,‘, the relative importance of the contribution of the canonical form (I) further ;3lcreases *The actual value of r(N7-C9) is likely to be even smaller than 1.166 A_ The position of C9 obtained from the high+ refinement may still be affected by the presence of the lone pair of electrons, whose contribution to the scattering of X-rays peak& well beyond sin e Ih = 0.55 A-‘.
31
Fig 6. Projection of the crysti plane, showing the environment
sticture of p-diisoqanobeuzene of the isxyano group. Short
onb the molecular intermolecular contacts
involving the isocyano group are: C9- - -Clti, 3.54 A; C9- - -C2ti and C9- - -C6G, 3.66 A; 3.79 A; C9-. -C6= and C9- - -C2iy, 3-82 A;C9- -CIOv and c9- - -C5”* and C9- - .C3’“. C9- - -CIOvi. 3.70 A; C9- - -N8” and Cg- - -N@, 3.72 A; C9- - 425” and C9- - -csyi, 3-67 A; C9- - -H5Y and C9- - -:-UyI, 2.78 A; N7- - -C4”, 3.55 A; N7- - -C3” and N7- - C5ti’, 3.66A;N7---C5tiandN7---C3 iv , 3.79 A. The symmetry operations relating the different molecules to molecule (i) (which is atr. y, z) are: (ii) -1+x, y, I, (iii) x, 1/2+y, 1/2+-t; (iv) x. -1/2+y, l/2+2; (v) -1+x, 1/2+y, l/2+2; (vi) -1+x, -1/2+y, 1/2+z. TABLE
6
Deviations
from
planarity
Atom Cl c2 N? c9 H2
in crystir=p-diisocysnobenzene’ Displacement
(A)
0.000 0.000 4.013 -0.020 -9.01
PDisplacements from the least-squares ring and etaadard deviations in the results of the conventional refinement
Standard
deviation
(A)
0.001 0.001 o-001 O-002 0.01 plane through the six carbon atoms of the benzene atomic positions irave been calculated from the
in going from the gaseous phase to the cry&& Indeed, the charge-transfer interaction between the benzene ring and the two adjacent isocyano groups
in the crystal cannot occur with forms (II) and (III), where the lobes of the IT* orbital of the N=C double bond are parallel to the molecular plane. In a p-disubstituted
benzene
ring with L&,
qmunek-y
the length or’ the
32
central C-C bonds may differ from that of the Cipm
c21ACKNOWLEDGEMENTS
We thank Mrs_ M5ri.a Kolonits for txperhental work, MB_ Clara Ma&ante and Mrs. M&a SzentgySrgyi for technical assistance. REFERENCES 1 M. Colapiefro, A Domenicmo, I. Hargittai, G. Portalone, Gy. Schultz and I. Torrini, paper presented at the Eighth European Crystallogmphic Meeting. Li*e. S-12 August, 1983, Absk.. p_ 167. 2 M Colapietro, A. Domeniczno. G. Portalone, Gy. Schulz and L Hargittai, J_ Mol. ShRct, 112 (1984) 141. 3 R_ Huhne, Acta Crysdlagr., 5 (1952 j 144. 4 (a) G. Portalone, Gy. Schultz, A Domenicano and I. Hargittai. J. Mel Struct., 118 (1984) 53, and references :herein. (b) M_ Colapietro, A. Domenicano, C. Marciante and G_ Portalone, 2. Naturforsch.. (1984) in press. and references therein. 5 R. G. A New and L_ E. Sutton, J_ Chem. Sot. (1932) 1415. 6 (2) L Hargittai, J_ Herr&B and M. Koloni& Prib. Tekh. Eksp.. (1972) 239. (b) J. Tremmel, M. Ko!onits and I. Hargittai. J. Phyn E, 10 (1977) 664. (c) I_ Hargiti, J_ Tremm el and M. Kolonits, Hung Sci. bna. 50 (1980) 31. 7 L Hargittai, J. Heri&& M Kolonits and Gy. Schultz, Rev. Sci. Instrum. 42 (1971) 546. 8 A Domenicsno. Gy. Schule M. Kolonits and I. Hargittai, J. Mol. Struct.. 53 (1979) 197. 9 D. W. J_ Cruickshaok, in J. S. Rollett (Ed.), Computing Methods in Crystallography, Pergamon Press, Oxford, 1965, p_ 112. 10 F. Di Riemo, A Domenicano and L. Riva di Sanseverino. Acta Crystdogr.. Sect B, 36 (1980) 586. 11 V. Schomaker and K. N. Trueblood, Acta Crystallogr., Sect. B. 24 (1968) 63. 12 W_ R. Busing and H. A. Levy, Acta Crystallogr., 17 (1964) 142. 13 M Hargittai and I. Hargittai, J_ Chem. Phys, 59 (1973) 2513. 14 A. Domenicano and P_ Murray-Rust. Tetrahedron L&t_, (1979) 2283. and J. Shorter (Edr), Correlation Analysis in 15 0. Exner, in N. B. Chapman Chemistry, Plenum Prem, New York, NY, 1978, pp. 439-540. 16 B. Rozmndai, B. Zelei and L Hargittai, J. Mel Struct, 95 (1982) 187. 17 B_ Bak, B_ P. Van Eijck and C. Kierkegasrd, J. Mol Struct.. 18 (1973) 429. 18 (a) C. van Rij and D. Britton. Acta Cqstallogr.. Sect. B, 33 (1977) 1301. (b) U. Driick and W. Littke, Aeta Qystallogr, Sect. B, 34 (1978) 3095. 19 C_ K Proct and J. D. Wrigb\ Angew. Chem. Int. Ed. Engl., 7 (1968) 659. 20 A Damiani, P_ De Santis, E Giglio. A M. Liquor& R. Puliti and A. Ripamonti. Acta CrystaRogr., 19 (1965) 340. 21 J. Gaultier and C. Hauw, Acta Crystakgr., 18 (1965) 179.
in The Netherlands
MOLECULAR STRUCTURE AND RING DISTORTIONS OF p-DIISCb CYANOBENZENE IN TEE GASEOUS PHASE AND IN T-HE CRYSTAL*
MARCELLO
coL&AmETRo
Institute of Stmctural Chemistry. ALDO
DOMENICANO**.
CNR,
GUSTAV0
I-00016
Monterotondo
PORTALGNE
and lNBS
Department of Chemisby and Institute of P?zarnuzccuticul Rome ‘La Sopienza ‘, I-00185 Rome (Italy) ISPVbLN
TGRRINI
Chemisfry.
Uniuersity
of
of Materials
Science.
The
Uniwrsity
SCHULTZ
Hungarivr Academy of Sciences. Research L-aboratov ment of Structural Studieq pf 1 I 7. H-l 43 1 Budarzt (Received
(Italy)
HARGITPAI**~*f*
Departments of Chemistry and Physics and InsBtute of Connecticuf Storrs. CT 06268 (U.S.A.) GYGRGY
Stazione
8 May
for Inoganic (Hung-)
Chemism.
Depcut-
1984)
The molecular structure of p-diisocyanobenzenr has been accurately determined in the gaseous phase (by electron diffraction) and in the crystal (by X-ray aystallography). The X-ray diffmction results have been corrected for the effects of solid state thermal motions and asphericities in valence eleclrons, so as to allow a comparison with gasphase rsulk In both phw the benzene ring is found to deviate appreciably from D, h symmetry: the most pronounced distortion is an increase to - 122” of the internal angle at the ipso carbon, e The geometry of the sulxtituent indicates that the dipolar form -fi& contributes to the electronic structure substantially more than does -N=C_ The following structural change occur on going from the free molecule to the crysta+l molecule: (i) an increase of Q from 121.7 2 0.2” to 122_2(3)“,t(ii) an increase of r(C-N) from 1.388 2 0.003 A to l-396(3) A, and (iii) a decrease of r(N&) from 1.176 = O-002 A to l-166(4) A. The consistent pattern of these changes indicates that the contribution of the dipolar form of the substituent is larger in the crystalhne than in the gaseous state. The crystals are composed of stacks of parallel molecules arranged in such a way that each benzene ring is sandwiched between two antiparallel isocyano groups, at a distance of 3.45 _&_ This arrangement appears to stabilize the dipolar form of the substituent. which has a v* orbital properly oriented to favour a charge-transfer interaction with a P orbital of the neighbouring benzene ring. *A preliminary report of this work has been presented elsewhere [ 1 I**To whom all correspondence should be addressed **+Viiting professor, on leave from the Hungarian Academy of Sciences.
0022-2860/84/$03.00
g) 1984
Elsevier
Science
Publishers
B-V-
Budapest
30
The effects of the crystal field on molecular struct-uze can best be ascertained by accurately determining the geometry of the same molecule in the gaseous and solid statis. We have recently studied the molecular structure of p-dicyanobenzene in the two states, by gas electron diffraction and X-ray crystallography, respectively [2]
_ THE ELECTRON DIFFRACTION STUDY
Experimental
and &ta
reduction
p-Diisocyanobenzene was prepared according to ref. 5, and purified by vacuum sublimation followed by recrystallization from ethyl ether/n-hexane. The electron diffraction patterns were taken in Budapest with a modified EG-100A apparatus [6], using the so-called membrane nozzle system [7] _ The nozzle temperature was about 111 “C!; nozzle-to-plate distances of about 50 and 19 cm were used. The treatment of the experimental data was the same as in ref. 8. The ranges of the intensity data used in the analysis were 2.900 < s S 14.000 A-’ and 9.25 < s < 36.00 A-‘, with data intervals As = 0.125 and 0.25 A-‘, respectively_ The total experimental intensities are available elsewhere [from the British Library Lending Division, Wetherby, Yorks., as Supplementary Publication No. SUP 26270 (11 pages)] _ The fmal molecular intensities are shown in Fig. l_ The esperimenti and theoretical radial distributions are presented in Fig 2_ They were calculated us&g au artificial damping factor equal to 0.002 A’. Theoretical values vere used in the 0.00 < SC 2_00A-’ region. The radial distribution closely resembles that obtained for p+Scyanobc?nzene [2] _ Structure analysis The least-squares method was applied to the mo!ecular intensities in the same manner as in refs. 2 and 8, assuming D = molecular symmew_ The best fit to the experimental data (R = 0.0321) was given by the molecular parameters presented in Table 1. Twenty-five independent parameters were varied in this refmement; among these were six geometrical parameters, five shrinkzg?s a.nd nine uncoupled amplitudes of vibration. Nine other amplitudes were coupled in three blocks and refined under the constraint of constant differences within each block (see Table 1). *An early two-dimensional study pletely inadequate for the purpoe
of the crystal structure of this compound of the present investigation
[3]
was com-
21
P-
-0
5
10
-24.. 15
c&(Nc)2
--^ 20
25
s-A-e+
30
5.h’
35
Fig_ 1_ Molecular intensity curves for the two camera ranges [(- - -) experimental; theoretical]. Also shown are the difference curves (experimental - tkoreticall
(-_)
Fig_ 2. Radial distribution CUI-VCS I(---) experimental; (-) theoretical]. The position of the most important distances is marked with vertical bars, the height of which is pmportional to the weight of the distances Also shown is the difference curve (experimental theoretical).
22
TABLE
1
Final molecular param r& (a) Distances
km,
and mewz amplibtdes
the electron diffrstior of vibration
study
(A)
Atomic P&
Multiplicity
‘*
I
Key to the coupling szbeme
Cl-C2 Cl-N7 N7-C9 c-H2
6 2 2 4 4
Cl - -. c4 C2---C5 c2- - - C6 N7---C2 N7---C3 N7---C4 NF---NS c9---Cl c9---c2 c9---c3 c9---c4 C9---N8 c9--- Cl0 Cl ---H2 Cl---H3 C2---H3 C2---H5 C2---H6 N7---H2 N7 - - - H3 C9---H2 C9---H3 H2---H3 H2---H5 H2---H6
1 2 2 4 4 2 1 2 4 4 2 2 1 4 4 4 4 4 4 4 4 4 2 2 2
O-0512(4) 0.0562 0.040(l) 0.077= 0.059(l) O-058(3) 0.073 O-059 0.067 O-069(2) O-075(3) O-081(9) O-057 0.110(3) O-097(3) O-071(5) 0_087(6) O.OSS(ZOj 0.100= 0.100= 0_100= 0.100= 0.100= 0 125= 0:125c 0.135= o-135= 0.140= 0.140= 0_140=
i i ii
Cl---C3
l-3980(4) 1.386(lP 1.1747(5) 1.083(3) 2.411(1)5 2_759(3F 2.814(2p 2442(2p 2401(l)b 3.674(l)b 4.145(2)b 5_531(3)b 2_561( l)b 3.456(4)b 4_773(4)b 5_304(5)b 6.675(7)b 7_852(23p 2.161(9)b 3.396(7)b 2157(10)b 3_897(4)b 3.421(7)b 2.639(15)b 4_553(lO)b 3_452(16)b 5.615(11)b 2489(29)b 4.979(7)b 4_313(17)b
(b) Angkr
c). dmerences
~cHl-C6 LC~-C~-H% A (C-NC)e 6(C9---C2) 6(C9---C3) &(C9---C4) 6(C9---NS) 6(C9---ClO)
between
bond
distances
(A).
Vake
Parameter (a)
and shrink.agesd
___ ul
iv
iv .._ ll1 .._
111 v
vi vii --_ 1u ___
VIII ix X
xi xii
(A)
121.74(11) 120.2(9) +J.O12(2) 0.008(4) 0. @25(4) O-017(6) 0.031(8) O-029(23)
‘Least-squares standard deviations are given in parentheses as units k the last digit bDependent parameter. cAsu.medd. dThe shrinkage is defined here as the decrease in a non-bonded distance when refmed as an independent parameter, as compared with the due calculated by employing geometrical constcaints. 'A (C-NC)= r(Cl-N7)-r(Cl-C2).
23
Many altemative values were tested for the differences between the amplitudes of vibration coupled in each block. The geometrical parameters of the heavy-atom skeleton were found to be virtuaUy insensitive to the choice of these differences, witi the noticeable exception of the parameter A(C-C) = r(Cl-C2) - r(C2-C3)_ Thus only a mean value of r(C-C) was eventually refined_* The value assumed for AZ = Z(C9 - - - Cl) - Z(C1 -- - C3) was found to affect appreciably some of the shrinkages and the C3<2-II2 angle. The shrinkages S(C9---C2), S(C9---C3), S(C9---C4), s(C9---N8) and 6(C9---ClO) were all successfully refined as independent parameters. 6(C9--C2) and 6(C9--C4) appeared to be somewhat sensitive to the conditions of the analysis, but in no case were they found to vanish or to lxcome negative. Refinement of shrinkages for the C9- - -Cl and N7 - - - N& distances was attempted, but this led to very small or negative values. Ignoring the shrinkage effect seriously worsened the fit (R = 0.0378) and caused the angles Q and C3-C2-H2 to increase by 0.4” and l-2’, respectively_ THE X-RAY DIFFRACTION STUDY Experimen
taaland data reduction
Colourless crystals suitable for X-ray diffraction work were grown by vacuum sublimation_ They were found to decompose rapidly in the X-ray beam unless sealed in 8 glass capillary. Accurate unit cell parameters were dete-mined by leastsquares techniques from the e-values of 37 reflexions, e-range i7-22” (MO KQ radiation), centred on the Syntex P2, difc%actometer of the Institute of Stxuctural Chemistry (Monterotondo Stazione). The content of the monoclinic unit cell is two molecules of pdiisocyanobenzene. Crystal data are reported in Table 2_ TABLE
2
Crystal dataa: pdiisocyanobenzene, C,H,N,, dec.). Monoclinic, space group A2/m, 2 = 2. a b C
*The asumptior; product
F.W.
4.772( 3) A 7.232(3) A 9.887(5) A of a(C-C)
= +O.Oll
128-14,
mp. 165-166-C
D, as from
the X-ray
K,
98_2a(5)” 337.7( 3) A” 1.260 g cm-
8 V
A,
(438-439
diffraction
study,
had only
A, as from tbe X-ray diffraction *The assumption of A(C<) = rO.O1l marginal effects on the I? factir and on the other geometrical parameter
study,
had oniy
24
Two different crystals were used for data collection. A rather large crystal, O-38 X O-63 X 0.70 mm3, was used to measure most of the intensities_ A smaller crystal (0.30 X O-35 X O-18 nun3) was used to obtain accurate intensities in the low-0 region. The intensity data were collected at room temperature (295-300 K) in the e-20 scan mode with graphite-monochromatized MO KQ radiation_ The scanning intenml was from 28 (KQ,) - 1.5” to 28 (Kc!*) + 1.7” for the larger crystal, and i?orn !i?d(K~,)- 1.2” to 28(Ka,) + 1.6’ for the smaller. The scanning rate was varied according to intensity, fiorn a minimum of 0.017” s-’ to a maximum of 0.49” s-l. The background was measured for l/6 of the scanning time at each end of the scan. The reciprocal space was explored from 0 = 1.5” to 35” for the larger crystal, and from 19 = 1.5” to 25” for the smaller one. Four standard reflexions monitored periodicaly did not indicate crystal decomposition. The intensities were corrected for Lorentz and polarization effects, but not for absorption [r(Mo -Ku) is 0.086 mm-’ ] _ For the larger c_rystal the reflexions measured were 1604, of which 834, having I > 3.5 cr. were considered as non-zero_ Both the hkl and i&l reflexions were measured. Averaging and merging [internal R(F,) = 0.0491 led to a set of 412 independent observations. For the smaller crystal 298 hkl and hxI reCexi0n.s with I > 8 or were used. The intensity data from the two crystals were put on the same scale using common reflexions [internal R(Fo) = 0.062] and merged into a single data set of 415 independent observations. Structure
determination
and refinement
The observed systematic extinctions (hkl, k + 1 = 2n + 1) are consistent with the space groups A 2, Am, A2/m. The Patterson synthesis led to an unambiguous solution in A2im_ ‘Ibe two molecules contained in the unit cell have site symmetry 2/m (C,); the mirror is perpendicular to -he benzene ring and contains the long axis of the molecule_ The atomic parameters produced by the Patterson synthesis were refined by least-squares techniques. -The hydrogen atom was located in a difference synthesis_ The final refinement was by full-ma& least-squares techniques, minimizing Cw(lFO I - klF,I)‘, with the non-hydrogen aroms treated anisotropically and the hydrogen atom isotropically_ Weights were given according to the empirical function w = l/(a f blFo I I clFO I’) 19, lo], with u = 4-5, b = 1-O and c = 0.44_ The atomic form factors were the same as in = ref. 4b. The number of observations per refined p ammeter was 415j32 13-O_ The final R and R, were 0.0333 and 0.0543, respectively, with all shifts less than 0.02 0. The final atomic coordinates are given in Table 3. Observed and calculated structure factors and anisotropic thermal parameters are available as supplementary material. A ‘high4 refinement’ was also carried out, using only reflexions with sin 19/x > 0.55 A-’ (0 > 2?), and treating the hydrogen atoms as in ref. 2.
25 TABLE Final
3 atomic
-rdinateP
la) Non-hydrogen
atomsb
Atom
‘,0.X
Cl
8314(3) 8304(5) 9149(2) 9139(5) 6609(a) 6614(S)
c2 N7
c9 (b) Hydrogen Atom H2
from
5193(4) 5168(i) atomd 10% 853(3)
the X-ray
diffraction
10.y
study
104
B c,c(-q=)
1005(2) 1004(2) 507(l) 5(39(3) 2044(1 j 204 l(2)
3.35 3.13 3-81 3.52 3.91 3.68
2699( 2) 2593(3)
4.86 4-50
1 O’y
10’2
B (a’)
280(2)
69(l)
28(3)
d
0 1676(l) 1680(l) 0 0 0 0
aLeast-s.quares standard deviations are given in parentheas units in the last digit_ The coordinates of C6 and H6 are obtained from those of C2 aud HZ respectively, by applying the symmetry operation x, y, z_ The coordinates of C3, C4, C5, H3. H5, N8 and Cl0 are obtained from those of C6. Cl, C2. H6, H2, N7 and C9, respectively. by applying the symmetry operation 2 -z, y. ---2. bFor each atom the values on the top line refer to the conventional refinement, those on the bottom line to the high-e refinement_ =Defiied as (B,B;B,j’“. where Bi = 8n’q (i = 1, 2. 3)_ The tYi are the r_m.r amplitudes of vibration along the principal axes of the thermal ellipsoid aFrom the conventional reflnemeut
Compared with the ‘conventional’ refinement based on aJl reflexions this refiiement gives atomic positions which are closer to nuclear posit-ions. The number of observations per refined parameter was 206/28 = 7.4; unit weights were given to all reflexions_ The final R and IL, were 0.0319 and 0.0363, respectively, with all shifts less than 0.01 u. The resulting atomic coordinates are presented in Table 3; anisotropic thermal parameters are available as supplementary material. Solid-state
thermal motions
The motion of the molecule in the crystal shows a clear anisotropy, with the major axes of all thermal ellipsoids roughly perpendicular to the molecular plane (Fig. 3)_ This s=sgests that the contribution corn internal vibrations is smaller than that tirn translational and librational rigid-body motions. We have thus carried out a thermal motion analysis in terms of rigid-body motions, using Schomaker and Trueblood’s approach [ll] _ The elements of the translational (T) and librational (L) tensors have been derived by least-squares techniques from the anisotiopic thermal paxamet-en U, (the eIements of the mean-square displacement tensor of each
26
Fig 3. An ORTEP drawing of thep-diisocyanobenzene molecule, showing the anisotropy of thermal motion in the crystal_ The thermal ellipsoids of the non-hydrogen atoms have been scaled to s 50% probability IeveL (a) Projection onto the plane of the benzene ring. (b) Projection onto the crystallographic mirror (hydrogen atoms have been omitted for clarity)_ Tbe drawing is based on the atomic p arameters from the high+ refinement
atom) of the high8 refinement. The agreement between observed and calculated U, values is good: R, = ~lAU,l/~IU,I = 0.0251.The r.m.s. values of the differences be~;ueen observed and calculated Uji’s, and of the estimated standard deviations aUti, are <@UJ’ > ‘I* = 0.00095 A’ and <(DU# > 11* = 0.00068 A*, respectively. The elements of the T and L tensors are available as supplementary material. Their eigenvalues and eigenvectors (which give the magnitudes and directions of the principal components) are given in Table 4 together with thou of the inertial tensor I. The librational motion is seen to be highly anisotropic, with the principal axes of L closely corresponding to those of I. As in p-dicyano’benzece [2], the largest oscillation
27 TABLE Analysis
4 of solid-state
(a) Eigerwalues
Tepsor T
L
I
thermal
motiona
and eigenvectom of the irans1atior.a~ Eigenvalues
T,
0.040
T, T, L, Lz L, 1,
O-036 0.035 0.0169 radl 0.0027 O-0015 722.4 anxu A=
1, I,
651.5 70.9
1ibmtionaI and inertiaIb tensors EigenvectorS 0.12
Al
0.00 0.99 O-76 0.00 O-65 0.72 0.06 -0-69
0.99 0.00 -0.12 -0.65 0.00 O-76 0.69 0.00 0.72
0.00 -1.00 0.00 O-00 1.00 0.00 o-00 1.00 0.00
(b) R.mr amplitudes of vibration along the principal axes of the translational and libmtional tensors Tensor Amplitudes (A) Tensor Amplitudes 0 T
O-20 0.19 0.19
L
i.4 3.0 22
aBased on the anisotropic thermal parameters from the high-e refinement. bHeavy-atom framework. The orthononnal reference system used in the analysis (a’, b’. c’ ) has the a’ and b’ apes coinciding with the a and b axes, respectively, of the monoclinic system (4
b, cl-
with the fact that no shrinkage of the Cl - - - C9 distance was observed in the electron diffraction study. It appears that the isocyano group in the present molecule is much more rigid than the cyvlo group in p-dicyanobenzene [2] _ Calculations for the X-ray and electron diffraction studies were carried out on the Univac 1100/82 computer of the University of Rome and on the HP 21MX minicomputer of the CNR Research Area (Monterotondo Stazione), using the same programs as in ref. 2. RESULTS
AND
DISCUSSION
The free molecule
The gas-phase structure of p-diisocyanobenzene, as obtained fi-om the electron diffraction study, is reported in Fig. 4d. The benzene ring is seen to deviate significantly from Ddh symmetry. The rather high value of the endocyclic angle at the ipso atom, a = 121.7 2 O-2”, is a clear indication of the strong u-electron withdrawing character of the isocyano group [14] _ In gaseous p-dicyanobenzene [2] the corresponding angle was found to be slightly, though perhaps not significantly, larger, a = 122.1 f 0.29 As the
28
Fig. 4. Molecular geometry of p-diisocyanobenzene, as determined by (a) X-ray crystallography, conventional refinement; (b) X-ray crystallography, high-0 refiiement; (c) as (b), after correction for libration; and (d) gas electron diffraction Bond distances are given in A, angk in degrees; the distance in (d) are rs valuex Only an average value of the ring C-C bond distances has been determined by elech-on diffraction. Here and throughout this paper total errors (estimated as in ref. 13) are given as error limits; leastsquaresstandard deviations are given in parenthesg as units in the last digit
group is generally considered to be more polar than the cyan0 a diEerence with the opposite sign might have been expected. gfonp*, The mean length of the ring C-C bonds in p-diisocyanobenzene is = 1.400 + 0.003 A, compared with 1.397 f 0.003 A in pr’g(C-c),, dicyanobenzene [2] _ Although the difference is not significant, the slightly larger value obtained in the present molecule is consistent with the smailer value of Q [lS] _ The N7-C9 bond length, rg = l-176 k O-002 A, is in the range expected for a triple bond, thus indicating that the canonical form (I) is the main contributor to the structure of the molecule_ Nevertheless, comparison with p-dicyanobenzene, r,(CEN) = 1-167 + 0.002 A [2], suggests that COPtzibutions from canonical forms like (II) and (III) may also be of importance.
isocyano
*The values of Taft’s inductive param et=. up are 0.67 for the isocyano group and 0.57 for the cyano group [ 15]_ It should be recognired, however. that these values originate from
solution studies (they have been derived from the pK% of and thus may not be transferable to the free molecules_
in water).
substituted acetic acids
29
(II
(D3
(IlIt1
The structure of the monosubstituted derivative, isocyanobenzene (phenylisocyanide) has been investig+l by microwave spectroscopy [17] _ Possible bond distances for the C-N=C lkagment have been derived under various alternative assumptions from the spectra of the parent compound and l+ee deuterated species. The valu+,es_obtained fall in rather wide ranges, = l-400-1.416 A and r,(N=C) = 1.136-1.160 A. The correro(C-N) sponding rs values from the present study are considerably different, 1.388 f 0.003 and 1.176 f 0.002 A, respectively. Needless to say, the molecular structure of isocyanobenzene calls for further examination. The crystal
structure
As with the cyan0 isomer, the crystals of pdiisocyanobenzene are composed of layers of parallel molecules. The molecular plane is close to the i02 plane, which gives by far the most intense reflexion of the diffraction pattern. The relative positions of the molecules in the two isomers are completely different, however_ In p-dicyanob enzene the benzene rings are partially overlapped, and the cyan0 groups take part in a complex system of intermolecular interactions that involve primarily antiparallel groups from adjacent molecules 12, lS]_ In p-diisocyanobenzene each benzene ring is sandwiched between two antiparallel isocyano groups, at a distance of 3-45 A (Fig 5). As in many molecular complexes of p-benzoquinone [19], this arrangement is favourable to a charg&ranxfer interaction, involving a 1~orbital of the benzene ring and a v * orbital of the adjacent isocyano group. The actual transfer of charge may be quite low, or even absent, however, as in the pyren~tctramethyluric acid 1:l complex [20] or in the ‘self complex’ of 1,4_naphthoquinone [21]. It is possible that the peculiar arrangement of planar molecules in these systems is required to maximize the contribution of the highly polar groups to the dispersion forces [19] _ The complete environment of the isocyano group in the crystals of pdiisocyanobenzene is shown in Fig. 6. The role of antiparallel interactions between adjacent functional groups is seen to be much less important here than inpdicyanobenzene [2] _
Fig 5. Schematic projection of the crystal structure of p-diisocyanobenzene The mclecules at O&Jhave been omittd for clarity.
down b_
The crystal .nolecule The molecular geometries of p-diisocyanobenzene obtained from the X-ray diffraction study are reported in Fig. 4a-c. The geometry most directly comparable with that fiorn the electron diffraction study is that of Fig. 4c, where the atomic cm&mates from the high4 refinement have been corrected for ‘Jle effects of libration. The correction is particularly important in the present case, as the crystal molecule undergoes a high-amplitude ‘rolling’ about the C9 - - - Cl0 axis. The site symmetry of the crystal molecule is Czh, but the benzene ring has Dlh symmetry, within the limits of experimental error (Table 5). The substituent is slightly bent out of the plane of the ring. A consistent pattern of minute structural changes occurs as the molecule of p-dikocyanobenzzne is frozen in the crystal. These include: (i) a 0.5” increase in the a angle, (ii) a 0.008 A increase of r(Cl-N7). and (iii) a 0.010 ZL decrease of r(N7-C9) _* Though small, these changes are beyond experimental error. When taken together, they indicate th_,‘, the relative importance of the contribution of the canonical form (I) further ;3lcreases *The actual value of r(N7-C9) is likely to be even smaller than 1.166 A_ The position of C9 obtained from the high+ refinement may still be affected by the presence of the lone pair of electrons, whose contribution to the scattering of X-rays peak& well beyond sin e Ih = 0.55 A-‘.
31
Fig 6. Projection of the crysti plane, showing the environment
sticture of p-diisoqanobeuzene of the isxyano group. Short
onb the molecular intermolecular contacts
involving the isocyano group are: C9- - -Clti, 3.54 A; C9- - -C2ti and C9- - -C6G, 3.66 A; 3.79 A; C9-. -C6= and C9- - -C2iy, 3-82 A;C9- -CIOv and c9- - -C5”* and C9- - .C3’“. C9- - -CIOvi. 3.70 A; C9- - -N8” and Cg- - -N@, 3.72 A; C9- - 425” and C9- - -csyi, 3-67 A; C9- - -H5Y and C9- - -:-UyI, 2.78 A; N7- - -C4”, 3.55 A; N7- - -C3” and N7- - C5ti’, 3.66A;N7---C5tiandN7---C3 iv , 3.79 A. The symmetry operations relating the different molecules to molecule (i) (which is atr. y, z) are: (ii) -1+x, y, I, (iii) x, 1/2+y, 1/2+-t; (iv) x. -1/2+y, l/2+2; (v) -1+x, 1/2+y, l/2+2; (vi) -1+x, -1/2+y, 1/2+z. TABLE
6
Deviations
from
planarity
Atom Cl c2 N? c9 H2
in crystir=p-diisocysnobenzene’ Displacement
(A)
0.000 0.000 4.013 -0.020 -9.01
PDisplacements from the least-squares ring and etaadard deviations in the results of the conventional refinement
Standard
deviation
(A)
0.001 0.001 o-001 O-002 0.01 plane through the six carbon atoms of the benzene atomic positions irave been calculated from the
in going from the gaseous phase to the cry&& Indeed, the charge-transfer interaction between the benzene ring and the two adjacent isocyano groups
in the crystal cannot occur with forms (II) and (III), where the lobes of the IT* orbital of the N=C double bond are parallel to the molecular plane. In a p-disubstituted
benzene
ring with L&,
qmunek-y
the length or’ the
32
central C-C bonds may differ from that of the Cipm
c21ACKNOWLEDGEMENTS
We thank Mrs_ M5ri.a Kolonits for txperhental work, MB_ Clara Ma&ante and Mrs. M&a SzentgySrgyi for technical assistance. REFERENCES 1 M. Colapiefro, A Domenicmo, I. Hargittai, G. Portalone, Gy. Schultz and I. Torrini, paper presented at the Eighth European Crystallogmphic Meeting. Li*e. S-12 August, 1983, Absk.. p_ 167. 2 M Colapietro, A. Domeniczno. G. Portalone, Gy. Schulz and L Hargittai, J_ Mol. ShRct, 112 (1984) 141. 3 R_ Huhne, Acta Crysdlagr., 5 (1952 j 144. 4 (a) G. Portalone, Gy. Schultz, A Domenicano and I. Hargittai. J. Mel Struct., 118 (1984) 53, and references :herein. (b) M_ Colapietro, A. Domenicano, C. Marciante and G_ Portalone, 2. Naturforsch.. (1984) in press. and references therein. 5 R. G. A New and L_ E. Sutton, J_ Chem. Sot. (1932) 1415. 6 (2) L Hargittai, J_ Herr&B and M. Koloni& Prib. Tekh. Eksp.. (1972) 239. (b) J. Tremmel, M. Ko!onits and I. Hargittai. J. Phyn E, 10 (1977) 664. (c) I_ Hargiti, J_ Tremm el and M. Kolonits, Hung Sci. bna. 50 (1980) 31. 7 L Hargittai, J. Heri&& M Kolonits and Gy. Schultz, Rev. Sci. Instrum. 42 (1971) 546. 8 A Domenicsno. Gy. Schule M. Kolonits and I. Hargittai, J. Mol. Struct.. 53 (1979) 197. 9 D. W. J_ Cruickshaok, in J. S. Rollett (Ed.), Computing Methods in Crystallography, Pergamon Press, Oxford, 1965, p_ 112. 10 F. Di Riemo, A Domenicano and L. Riva di Sanseverino. Acta Crystdogr.. Sect B, 36 (1980) 586. 11 V. Schomaker and K. N. Trueblood, Acta Crystallogr., Sect. B. 24 (1968) 63. 12 W_ R. Busing and H. A. Levy, Acta Crystallogr., 17 (1964) 142. 13 M Hargittai and I. Hargittai, J_ Chem. Phys, 59 (1973) 2513. 14 A. Domenicano and P_ Murray-Rust. Tetrahedron L&t_, (1979) 2283. and J. Shorter (Edr), Correlation Analysis in 15 0. Exner, in N. B. Chapman Chemistry, Plenum Prem, New York, NY, 1978, pp. 439-540. 16 B. Rozmndai, B. Zelei and L Hargittai, J. Mel Struct, 95 (1982) 187. 17 B_ Bak, B_ P. Van Eijck and C. Kierkegasrd, J. Mol Struct.. 18 (1973) 429. 18 (a) C. van Rij and D. Britton. Acta Cqstallogr.. Sect. B, 33 (1977) 1301. (b) U. Driick and W. Littke, Aeta Qystallogr, Sect. B, 34 (1978) 3095. 19 C_ K Proct and J. D. Wrigb\ Angew. Chem. Int. Ed. Engl., 7 (1968) 659. 20 A Damiani, P_ De Santis, E Giglio. A M. Liquor& R. Puliti and A. Ripamonti. Acta CrystaRogr., 19 (1965) 340. 21 J. Gaultier and C. Hauw, Acta Crystakgr., 18 (1965) 179.