Structures and conformations of carbonyl isocyanates and carbonyl azides. An experimental and theoretical investigation

Journal of Molecular Structure, 291 (1993) 197-209 Elsevier Science Publishers B.V., Amsterdam

197

Structures and conformations of carbonyl isocyanates and carbonyl azides. An experimental and theoretical investigation Hans-Georg Mack”, Carlos 0. Della VCdovab and Helge Willner” ‘Institut ftir Physikalische und Theoretische Chemie, Universitiit Tiibingen, D-7400 Tiibingen (Gefmany) bFacultad de Ciencias Exactas, Universidad National de La Plats, Departamento de Quimica, Quimica Inorganica, 1900 La Plata (Argentina) ‘Institut ftir Anorganische Chemie, Universitiit Hannover, D-3000 Hannover (Germany)

(Received 4 September 1992) Abstract The geometric structures and conformational compositions of fluorocarbonyl isocyanate, FC(O)NCO, and fluorocarbonyl azide, FC(O)N,, were studied by gas electron diffraction, vibrational spectroscopy and ab initio calculations. Both compounds exist in the gas phase as a mixture of two planar conformers (C, symmetry), with the cis form (carbonyl C = 0 double bond in the cis position with respect to the N = C or N = N bond) being lower in energy than the trans rotamer. From the experimental scattering intensities the following geometric parameters (ra distances and L,, angles with 3a uncertainties) were derived for the predominating cis structures. FC(O)NCO, 75(12)%cis:C=O(isocyanate) = l.l54(8)A,C=O(carbonyl) = l.l92(8)&N=C = 1.215(9)f$C-F = 1.320 A (not refined), N-C = 1.388(4) A, C-N=C = 125.9(11)0, N-C = 0 = 128.7(6)0, N-C-F = 107.8(8)0 and N=C=O = 174.6(25)“. FC(O)N,, 90(10)% cis: N@=N, = 1.124(5) A, N,=N, = 1.246(5) A, C=O = 1.192 A (not refined), C-F = 1.324(3) A, N,-C = 1.390(4) A, C-N,=N, = 110.9(8)0, N,-C=O = 129.3(4)0, N,-C-F = 107.4( 15)o and N, = N, = N, = 170.7(27)0. The experimentally obtained energy differences between the two conformers of FC(O)NCO (AE(ED) = E,,,, - E,, = 0.7(3) kcal mol-‘, AE(IR) = 0..4(2) kcal mol-I, AE (Raman) = 0.8(3) kcal mol-I) and of FC(O)N, (AE(ED) = 1.3(6) kcal mol-‘, AE(IR) = 1.5(4) kcal mol-‘, AE(Raman) = 1.3(4) kcal mol-I) can be reproduced within their error limits by MP2/6-3lG*//HF/6_3lG* and MP4SDTQ/6-3lG*//HF/6-3lG* calculations. HF/6-31G* and MP2/6-31G* approximations overestimate these values. Corresponding ab initio calculations were performed for the parent compounds HC(O)NCO and HC(O)N,, which are not stable and cannot be studied in the gas phase.

\

Introduction

/

The conformation of molecules with vicinal 1,3double bonds, i.e. trams, gauche or cis positions of the double bonds relative to each other, depends on the types of double bonds A = X and B = Y, on the substituents on A and B (atoms, groups or lone pairs), and on the terminal groups or atoms X and Y. In most parent compounds studied so far, such as 1,3-butadiene (vicinal C = C/C = C double Correspondence to: Dr. H.-G. Mack, Institut fur Physikalische und Theoretische Chemie, Universitlt Tubingen, D-7400 Tiibingen, Germany.

\

B=Y

X’A

\ B

+

-A

:::I

B=Y

y

/

\

kx

trans

gauche

/ =

-A\X cis

bonds) [l], glyoxal (C=O/C=O) [2], acrolein (C = C/C = 0) [3] or formaldazine (N = C/N = C) [4], the ground state structure is planar trans. Substituted derivatives, however, such as perchloro- [5] and pertluoro-1,3-butadiene [6] or the brominated species of formaldazine [7j prefer non-planar gauche structures. Only planar structures, i.e. cis conformations or mixtures of cis and trans

0022-2860/93/%06.00 0 1993 Elsevier Science Publishers B.V. All rights reserved.

198

forms, were determined for carbonyl isocyanates (C = O/N= C). In the case of acetyl isocyanate, CH,C(O)NCO, only the cis conformer is observed [8]; for chlorocarbonyl isocyanate, ClC(O)NCO, a mixture of trans and cis forms with a ratio of about 3 : 1 is found [9]. According to the interpretation of the microwave spectra, vicinal C=C and N= C bonds in vinyl isocyanate [lo] prefer the trans position with respect to each other. However, the predominant form of vinyl azide (C = C/N = N) is the cis rotamer [l 11. In this work we investigate the geometric structures and conformational properties of fluorocarbonyl isocyanate, FC(O)NCO, and the corresponding carbonyl azide, FC(O)N,, by gas-phase electron diffraction and vibrational analyses. These experiments are supplemented by ab initio calculations at various levels of sophistication in order to check the reliability of different computational methods for predicting conformational properties of compounds of this type. A previous investigation of ClC(O)NCO [9] demonstrated that standard computational methods fail in predicting the correct ground-state conformation for this molecule. In addition to these studies of the fluorocarbonyl derivatives, we performed ab initio calculations for the parent compounds HC(O)NCO and HC(O)N, which, to our knowledge, are not stable and therefore cannot be studied experimentally. Experimental Fluorocarbonyl isocyanate, FC(O)NCO, was prepared by reaction of chlorocarbonyl isocyanate, ClC(O)NCO, and XeF,. For more preparative details see ref. 12. Raman spectra between 4000 and 200 cm-’ were obtained with a Perkin-Elmer 1760 spectrometer equipped with a Nd : YAG laser. The 1064nm line was used for excitation. The spectra were recorded at room temperature and the resolution was 4cm-‘ . IR spectra between 4000 and 400cm-’ were obtained with an FT Bruker IFS 85 spectrometer (resolution 1 cm-‘). A lowtemperature matrix spectrum was recorded in a

H.-G.

Mack

et a/./J. Mol.

Strut.,

291 (1993)

197-209

cryogenic system with an approximate compound/ Ar ratio of 1: 1000. The continuous deposition technique was employed. Fluorocarbonyl axidea, FC(O)N, , was prepared by reacting dry NaN, (0.5mmol) with FC(O)Cl (1.5 mmol) in a sealed glass capillary tube (6 x 1.5 mm, 200mm long) at room temperature for 3 days. Three batches were collected in a vacuum apparatus and purified by repeated trap-to-trap condensation (- 60”, - loo’, - 196°C). The product at - 100°C was checked for purity by IR spectroscopy. The synthesis of the isotopically labeled samples is described elsewhere [13]. Raman spectra of the liquid compound in the spectral range 3000 to 100 cm-’ (resolution 3 cm-‘) were obtained with a Varian spectrometer Cary 82, equipped with an Ar+-ion laser (514.5nm, 500mW). Gas-phase IR spectra were recorded using the FT-IR spectrometer MX-1 (Nicolet) operating between 4800 and 400cm-’ with a resolution of 2cm-‘ . Matrix IR spectra were obtained with the FT-IR spectrometer IFS66v (Bruker) operating in reflection mode between 4000 and 400 cm-’ (resolution 1 cm-‘). Samples with an approximate ratio FC(O)N, : Ne of 1: 1000 were deposited on a metal mirror at 5 K applying the continuous deposition technique. The electron diffraction intensities were recorded with a Balzers gas diffractograph KD-G2 [14] at two camera (nozzle-to-plate) distances (25 and 50 cm) with an accelerating voltage of about 60 kV. The electron wavelength was determined with ZnO diffraction patterns in each experiment. The sample reservoirs were kept at -4O’C (FC(O)NCO) and -35°C (FC(O)N,), respectively; the inlet system and nozzle were maintained at room temperature. The camera pressure did not exceed lo-‘Torr during the experiments. The exposure times were 5-7 s (FC(O)NCO) and 7-9s (FC(O)N,) for the long camera distance, and 22-25s (FC(O)NCO) and 24-27 s (FC(O)N,) for the short distance. Two

“Caution! Fluorocarbonyl azide is explosive and should be handled only with proper safety precautions and in millimolar quantities.

H.-G. Mack et al./J. Mol. Struct..291(1993) 197-209

199

I

0

8

5

10

15

20

25

30

35

s/A-’ Fig. 1. Experimental (. . . .) and calculated (-) molecular scattering intensities and differences for FC(O)NCO.

Fig. 2. Experimental c. . .) and calculated (-) molecular scattering intensities and differences for FC(O)N,.

photographic plates for each compound and each camera distance were analyzed by the usual procedures [ 151.Numerical values for the total scattering intensities for both compounds in the s-ranges 2-18 A-’ and 8-35 A-’ in steps of As = 0.2A-’ are available”. The averaged molecular intensities are presented in Figs. 1 (FC(O)NCO) and 2

experimental structures (see below) demonstrate that both cis and trans rotamers are near-symmetric prolate tops (with asymmetry parameters rc = (2B - A - C)/(A - C) x -0.92). The separations between P- and R-branches of the expected A-type (PQR contour), B-type (two peaks) and AB hybrid IR bands for the in-plane modes (A’ species), and those of the C-type IR bands (PQR contour with prominent Q-branch) for the out-of-plane and NC0 deformation modes (A” species) were obtained according to Seth-Paul [16]. The results of these calculations are 12.8, 10.7 and 19.3~~~ for the separation between the P- and R-branches of the A, Band C bands of the cis form, and 12.7, 10.6 and 19.1 cm-’ for the trans conformer, respectively. For the AB hybrid bands such a separation is limited to the range of values calculated for the “pure” A and B bands. These calculated values are in good agreement with the experimental frequencies (Table 1).

(FC(O)N, ). Vibrational analysis of FC(O)NCO Spectra Table 1 presents IR-gas, IR-matrix and Raman-liquid vibrational frequencies together with a tentative assignment. The torsional vibrations are expected to be far below 400 cm-‘, which is the limit of the spectrometer employed. This Table also includes IR data for the compound isolated in an Ar matrix at low temperatures. From the vibrational spectra a conformational equilibrium of two planar structures with C, symmetry is expected. The envelope of the two carbonyl bands in the IR spectrum of the vapor points to the existence of cis and trans conformers (with respect to the relative positions of the vicinal C = O/N = C double bonds). The rotational constants calculated from the aData deposited with B.L.L.D. as Publication number SUP 26465 (4 pages).

Supplementary

Assignments The fundamental vibrational modes of FC(O)NCO were assigned by comparison with related molecules, taking characteristic wavenumbers and the contours of the vapor-phase IR bands into account. The proposed assignment was confirmed by applying the sum rule [17]. For both planar conformers the 3N - 6 = 12 normal modes of

H.-G. Mack et al./J. Mol. Struct., 291 (1993) 197-209

200 TABLE 1 Vibrational

data for FC(O)NCO Raman

IR Vapor (cm-‘) 3673 3141 2298 2262 1880 1846

contour Av(PR)

Intensity

Matrix-Ar

Assignment” Rel. int.

Liquid (a-‘)

Cm-‘) W

VI +

v3

VW

VI +

vs

AB(14) AB( 11)

m

v,

A(I3) 803)

s

1436

A(121

s

1232 1163 1112 885

A(l2) A(121 AB(12)

W

m

W3)

W

c

W

149 685 616 562 480

VS

s

s

W

C

W

803)

VW

2258 1843 1827 1453 1449 1212 1146 1109 880 873 145 686 679

vibration can be classified as 2N - 3 = 9A’ (I’, to vg, in-plane) modes and N - 3 = 3A” (vIOto vu, out-of-plane) modes, all of them active in the IR and Raman. The A’ and A” modes are expected to be polarized and depolarized, respectively, in the Raman spectra. Only the former were observed. The vibrational spectra show bands which could be assigned to both conformers. The strong bands centered at 1880 and 1846 cm-’ in the IR spectrum of the vapor can immediately be assigned to the C = 0 stretching modes of the trans and cis form, respectively. The C = 0 vibration of the cis rotamer possesses a B-type contour and that of the trans form a near A contour. Figure 3 presents the molecular models for the cis and tram forms of FC(O)NCO together with their corresponding principal axes. We can deduce from this Figure that the above-mentioned behavior is expected for the cis and trans conformers. This splitting of the C = 0

2254 1853 1826

W

vt v,,(NCO)

S

v2

VS

v2 v(CO)

1442

S

v3 v,(NCO)

trans

?O +

v8

v, VP)

1121 886 853

trans

W

v,

vs vs

~5 v(NC) vg tram vlo

Lp

v6 6

-605

554 525 498

W-W

C(O)F

~$1 4x,,

560

W

“Assigned wavenumbers are for the predominant cis conformer. as = asymmetric, oop = out-of-plane, ip = in-plane.

trans

(NC01

m

~76, (NCO)

W

vg

m

vg P C(O)F

v = stretching, p = rocking, 6 = deformation,

tram

s = symmetric,

vibrations into two bands (Av = 34cm-‘) is reproduced by ab initio calculations (HF/6-31G*, see below), which result in Av = 50 cm-’ . In agreement with the experiment the calculations predict the C=O vibration of the trans isomer at a higher frequency (2131 cm-‘) than that of the cis form (2081 cm-‘). The Raman spectra of the liquid also

cis

\

trans

Fig. 3. Molecular models with the principal axes and atom labeling for FC(O)NCO.

H.-G. Mack et al/J. Mol. Struct.. 291 (1993) 197-209

201

possesses two bands in the C = 0 stretching region, which correspond to the trans (1853 cm-‘) and cis (1826 cm-‘) conformers. The other assignments are given in Table 1. Using only split bands, which show conformational character, application of the sum rule results in 1.03 x 10’ and 1.OOx 10’ for the trans and cis form, respectively, thus confirming the proposed assignments. Conformational composition The band intensity ratio of the carbonyl vibrations (cis: trans) in the IR spectrum is 1 : 1. For the C =0 absorbances (squared dipole moment derivatives) between the cis and trans conformers, ab initio calculations (HF/6-31G*) result in a ratio of 588 : 1080 = 0.54. If this ratio is taken into account a conformational composition of 65( lo)% cis and 35(10)% trans is obtained. The error limits are due to an estimated uncertainty in the calculated ratio of the absorbancesa. The experimental Raman intensity ratio (cis : trans) for the C = 0 vibrations (Table 1) is 1.39. If, again, the theoretical ratio for the Raman activities (cis : trans = 0.38) is applied, a composition of the mixture of 78(1O)O/ cis and 22(10)% trans is derived. Thus, using the theoretical ratios for IR absorbances and Raman activities, the vibrational spectra result in the following values for the differences in the free energies between the trans and cis conformers: AG(IR) = G,,,,, - Gcis= 0.4(2)kcal mall’ and AG(Ra) = 0.8(3)kcal mall’ . Vibrational analysis of FC(0)N3 Spectra In Table 2 all observed band positions together with their assignments are collected. The expected gas band contours should be similar to those of the isoelectronic FC(O)NCO and the respective descriptions are also included in this Table. Bands of the less stable trans isomer were determined by cooling “These preliminary values for the conformational position were reported previously in refs. 8 and 12.

com-

(x 200 K) and heating (x 370 K) the nozzle during deposition of the matrix. All weak bands, which increase in intensity by increasing the temperature of the nozzle, were assigned to the trans conformer. Assignments The 12 fundamental vibrational modes of FC(O)N, were assigned by comparison with the modes of FC(O)Cl [181and of X-N3 molecules [ 193; corresponding bands of isotopically labeled species were compared and the contours of the vaporphase IR bands taken into account. For the 9A’ modes (v, to vg), A-, B- or AB-hybrid-type IR bands are expected, and for the three A” modes (vg to vu), C-type IR bands are predicted (see above). In the IR spectrum of the vapor the two bands at 745 and 565 cm-’ exhibit prominent Q-branches. Consequently, they are assigned to the A” modes &,,, C(O)F and N,, respectively. The remaining A” mode (torsional motion) can be assigned to the band in the Raman spectrum at 160cm-‘. Altematively, this band may be due to the bending mode vg of the trans conformer of FC(O)N,. By analogy with the bending mode at the lowest frequency of other X-N, molecules [ 191,the very strong Raman band at 188cm-’ is assigned to vg (SN,-C(O)F). The next two A’ deformation modes v7 and vs are strongly coupled. This leads to a shift of vS (6cN,) to an unusually low frequency and to an enhancement of the Raman intensity. The isotopic frequencies of OL-and w-‘~N labeled FC(O)N, for v7 cannot be distinguished because only mixtures of both isotopomers were available. The same situation is true for the “N frequencies of some other modes. For the straightforward assignments of the remaining A’ fundamentals, see Table 2. The fundamentals v3 and v,,, are in strong Fermi-resonance with (vg + v7) and (vg + v,,), respectively; the assignment of these and all other combinations are unambiguous taking the isotopically labeled FC(0)Ng species into account. Conformational composition For the trans form, six fundamental modes were detected, but only the v2 band has been used to

H.-G. Mack et al./J. Mol. Struct., 291 (1993) 197-209

202 TABLE

2

Vibrational

data for FC(O)N,

IR

Raman Matrix-Ne

Gas-phase Vapor

Contour

(cm-r)

Av(PR)

Rel. int.

3638

B(13) AB(11.5)

VW VW VW VW

3347 3128 2153 2332 2193 2160

AB(11.5) AB(13) AB(12.5) AB(12.5)

3632

3544

3564

3346

3339

3322

3316

3131

3334 3125

VI +

V5

2706

3100 2741

3110

2152

3113 2105

2751

Vz +

V5

2330 2194

2309

2321 2189

2319

VS W

2161

2086

A(13)

W

2086

1872

A(15)

m

1868

1829

B(13) B(12.5)

VS

B(13)

1455 1344

Rel. int

‘80

2191

15N8

Liquid (cm-‘) “C

W

Nat.b

15NY

36:2

2183

2v2 VI + v4

2308 2178

2195

vs

VI VdN3)

2150 2085

sh

v,

W

sh

vq + v5 v2 trans

VS

V2V (CO)

2013

2015

2060

2011

1782

1792

1826

VW

1826 1459

W

1347

1332

1441 1342

1443 1338

1821 1456

W

1260

2%

1850 1808

v,?

vs

1247 1236

1235

1226

vs

1217

1238

1231

1270 1250

1169

A(14)

ms

1167

1152

1165

1161

1165

W

1094

A(13.5)

850

145

C(l8)

ms VW

930 851

W

749

m

741

VW

129

1120 921

919 745

L

929

915 Y

/

VW

sh m VW

933 859

vs m

140

sh

139

120

131

146

V7

V.4 +

A A

929

trans

V5 +

1333

1236

1246

Assignment”

V9

trans

V6+ Vl v3v,,N-C(O)F V4VsPJ3) v4

trans

v5v, N-C(O)F v5 trans v9 + VII VIO+ &C(G)F vIOtrans

713

A(14)

W

706

705

704

703

713

S

v,SC(O)F

565 530

C(l8) B(13.5)

W

561 530

567 530

567

564

554

W

V,,4,o,(N,)

523

529

W

531

S

482 188

m vs

160

sh

V,K(OF V&N)

v,6N,-C(O)F v,,rN,-C(O)F (or trans)

“See Table 1. bNatural isotopic species.

estimate the conformational composition. The difference between the v1 vibrations of the cis and trans conformers is 42 cn-’ , in accordance with the The (Av = 60cm-‘). ab initio calculations cis : trans ratio of the v2 band intensities of the gas-phase and matrix spectra is 7.2. Taking the theoretical ratio of the absorbances (cis : trans = 0.55) into account, this yields a composition of 93(5)% cis and 7(5)O/, trans conformers. The experimental

Raman intensity ratio for the v2mode, measured at room temperature, is cis : trans = 4.8. Taking the theoretical ratio for the Raman activities (cis : trans = 0.51) into account, this leads to a conformational composition of 90(5)O/, cis and 10(5)% trans. The vibrational data, thus, yield free energy differences of AG(IR) =GtranS- Gcis= 1.5(4)kcalmol-’ and of AG(Ra) = 1,3(4)kcalmol-‘, respectively.

203

H.-G. Mack et al./J. Mol. Struct., 291 (1993) 197-209

trans

CiS

Exp.

0

1

3

2

4

5

R/A Fig. 4. Calculated (tram and cis) and experimental radial distribution functions, and difference curve (below) for FC(O)NCO. The positions of interatomic distances of the main conformer (cis) are indicated by vertical bars.

Electron diffraction analysis FC(0) NC0 Model calculations demonstrate that the experimental radial distribution function (see Fig. 4) cannot be reproduced with one single conformer. If bond distances and angles from analogous compounds are used, the isolated peak at about 4.5A can only be fitted with planar conformers. This peak corresponds to the Fe. ‘Oi non-bonded distance in the cis confo~er or to the O;**Oi distance of the trans form (for atom labeling see Fig. 3). For gauche conformations, the longest non-bonded distances (Fe* .Oi or 0; **Oi) would

be shorter than 4.5A. Such structures can be excluded on the basis of the experimental radial distribution curve. Non-planar conformations can also be ruled out by the results of the ab initio calculations (see below). The calculated radial distribution functions for the cis and trans rotamers of FC(O)NCO are presented in Fig. 4. The model curves show distinctive differences in the ranges 2.5-3.8& although oxygen and fluorine possess very similar scattering amplitudes. Comparison of the two calculated curves with the experimental radial distribution function clearly illustrates the predominance of the cis conformer. The preliminary geometric parameters derived from the experimental radial distribution curve and the relative contributions of the two isomers were refined by a least-squares analysis of the molecular scattering intensities. For this purpose, scattering amplitudes and phases reported by Haase [20] were used and a diagonal weight matrix was applied to the intensities. As a starting value for the composition of the mixture, the result of the analysis of the IR spectra (65% cis and 35% trans, see above) was used. In the first refinements, all geometric parameters for the cis and trans structures were assumed to be equal, except for the vibrational amplitudes for those distances that depend upon the molecular confo~ation. O;*-X and F.-*X amplitudes (X = Ci, Oi) of the cis form were interchanged in the trans conformer. Amplitudes for similar distances were grouped together and the constraints are evident from Table 3 (interatomic distances and vibrational amplitudes are given only for the predominant cis form). Ab initio calculations at the HF/6-31G* level predict marked differences in some geometric parameters of the two isomers, especially for the C-F distance (1.3OOA (cis) and 1.320 A (trans)) and the C-N = Ci angle (125.4” (cis) and 129.0” (trans)). The results of many investigations demonstrate that such calculated structural differences for very similar molecules or isomers of the same com~unds are highly reliable and, to a great extent, independent of the basis set applied [21]. For this reason, we introduced these theoretical differences for the two conformers of

204

H.-G. Mack ef al./J. Mol. Struct., 291 (1993) 197-209

TABLE 3 Results of electron diffraction analysis and ab initio calculations (HF/6-31G*) for FC(O)NCO (for atom labeling see Fig. 3) Geometric Bond

parameters ED=

Ab initio

Cis

Cis

N=C, C,-F N-C,

Mb) C,-N=C,

0.038b 1.173(4)

p,

1.154(8) 1.192&j 1.215 d 9) 1.32 1.388(4)

p2

125.9(11) 174.6(25) ;;;;$;

N=C, =O, N-C, = 0, N-Cc-F 0, =C,-Fe

123:5(10)

%

75112)

p3 p4

ps P6

P?

1.152 0.038 1.133 1.171 I.216 1.300 1.376

FC(O)OOC(O)F [22]. With these assumptions seven geometric parameters, seven vibrational amplitudes and the confo~er ratio were refined simultaneously. The following correlation coefficients had values larger than /0.6]: p, /pz = - 0.94, PJP~, = -0.62, Pi lb = 0.87, p&

Tram 1.149 0.028 1.135 1.163 1.212 1.320 1.374

125.4 173.8 128.0 109.4 122.6

129.0 174.0 126.3 111.6 122.1

94”

6

~41~s = - 0.62, = - 0.90 and p&

~71~6 = 0.61, = - 0.66 (for

labeling of the geometric parameters pi and vibrational amplitudes I, see Table 3). The geometric parameters for the more stable cis conformer together with the ab initio results (for the cis and trans forms) and the vibrational amplitudes (for the cis form only) are collected in Table 3.

Interatomic distances and vibrational amplitudes for the cis form Distance (A) q-0,

G-0, N-C,

C,-F N-C, N. .F O;,.F c; . C, N. .O, N. .Oi 0; . C, F. . Ci c;..o, 0;. .o, F. . .o,

1.16 1.19 1.22 1.32 1.39 1 2.19 2.21 1 2.32 2.32 2.37 i 2.84 3.36 3.41 3.72 4.51

Amplitude 0.044(4)

1,

0.041(4)

12

0.056’ 0.058(4)

I3

0.093( 11)

14

0.086(7)

1,

0.123(16) 0.081(10)

k 1,

“With 30 uncertainties. bAb initio value. cDependent parameter. dNot refined. ‘Calculated from the theoretical AE (= E,,, - E,,) value. ‘

FC(O)NCO as additional ~nstraints in the final refinements. Hereby, the agreement factors for both camera distances decreased and bond angles for the prevailing cis rotamer changed slightly. The C-F distance, which causes high correlations in the least-squares refinement, was set to the value in

0

1

2

3

I

‘

4

5

RIA Fig. 5. Calculated (tram and cis) and experimental radial distribution functions, and difference curve (below) for . The positions of interatomic distances of the main conformer (cis) are indicated by vertical bars.

205

H.-G. h4ack et al/J. Mol. Struct.. 291 (1993) 197-209

FC(o)N,

TABLE 4

The ex~rimental radial dist~bution function for this compound (Fig. 5) can be reproduced reasonably well only with a planar cis conformer. Molecular models for the cis and trans form together with atom labeling are shown in Fig. 6. The agreement between expe~mental and calculated radial distribution curves is slightly improved, if a small contribution of a planar trans form is added, as derived from the IR spectra. No non-planar gauche structures could be observed, in agreement with the ab initio calculations (see below). The prelimina~ structural model for the cis rotamer was then refined by a least-squares procedure of the molecular intensities. Application of least-squares analysis was analogous to that for FC(O)NCO. The C=O distance could not be refined because of high correlations and its value was set to that in FC(O)NCO. The vibrational amplitudes for the bonded distances N,= N,, C =0 and N, =N, were fixed to spectroscopic values for CF3 N3 [23]; the amplitudes for the C-F, N,-C and the non-bonded NE-*-N, distance caused high correlations in the least-squares procedure and, therefore, could not be refined. Further constraints are evident from Table 4. With these assumptions eight geometric parameters, four vibrational amplitudes and the confo~ation~ composition were refined simultaneously. The final results (experimental geometric parameters and vibrational amplitudes for the prevailing cis form as well as the calculated geometries for both conformers) are given in Table 4. The following

Results of electron diffraction analysis and ab initio ~lculations (HF~6-3lG*) for FC(O)N, (for atom labeling see Fig. 6) Geometric Bond

parameters ED”

Ab initio

Cis

Cis

Trans

r, (4 N#=N,

1.124(5)

N, =N, c=o C-F N--C

1.246(5)

;:

l.192b 1.324(3) 1.3%(4)

pS

Udeg) C-N,, = N, N,=N,-N, N,-C=O N,-C-F O=C-F

1 l&9(8) 170.7(27) 129.3(4) 107.4(15) 123.3(16)

%

90(10)

Interatomic cis form

%-Vu

ps ;

ps

distances and vibrational

1.088 1.262 1.165 1.325 1.379

110.6 173.7 128.9 108.2 122.9

114.4 172.7 125.0 113.0 122.0

z-99


amplitudes

Distance

Amplitude

1.12

0.03P

c-o

for the

o.042b

K-N, C-F N,-C C ’ .N, N;..F 0. . .F N; .O N; ’ .N, 0. . .NB C- . -N, F* . .N, 0. . .N, F,. .N,

P4

1.086 1.267 1.175 1.302 1.378

1.32 1.39 2.17 2.19 2.22 2.33 1 2.36 2.60 3.21 3.30 3.38 4.40

O&W

0.054(5)

1,

0.05sb 0.105(13)

12

0.081(9}

5

0.084( 14)

14

aWith 30 un~~ainties. b~pendent parameter. ‘Not refined.

VJ

cis

tram

Fig. 6. Molecular models and atom labeling for FC(O)N,.

correlation coefficients possess values larger than 10.61:p5/p6 = - 0.77 and p,/l, = 0.75 (for numbering of the geometric parameters pi and vibrational amplitudes & see Table 4). The best agreement

206

H.-G. Mack et al.lJ. Mol. Struct., 291 (1993) 197-209

MP4SDTQ single-point calculations with the corresponding HF-optimized geometries were carried out. The relative energies of the cis and trans forms, which represent the only stable structures on the energy hypersurface, as well as the barriers to rotation are collected in Table 5. For both molecules the calculations predict the trans conformer to be higher in energy than the cis rotamer. The energy differences AE( = E,,,, - Ecis) decrease upon inclusion of electron correlation. In additional calculations the cis and trans conformers of FC(O)NCO were optimized in the MP2 approximation. Surprisingly, this resulted in the same value for AE( = 1.7 kcalmoll’) as the Hartree-Fock calculations. For this reason, corresponding calculations for FC(0)N3 were not performed. In the HF approximation the barrier to rotation in FC(O)N, is about 10 kcal mol-’ higher than that in FC(O)NCO. This value is reduced to about 6 kcal mol-’ if electron correlation effects are taken into account. This big difference in the barrier heights indicates that conjugation between the carbonyl group and the N3 group in the azide compound is stronger than that between the C = 0 and the NC0 group in the isocyanate. The potential curves (HF/6-3 1G*) for internal rotation are shown

factors were obtained for a 90( lo)% planar cis and 10(lo)% trans composition. Ab initio calculations All calculations were performed with the program [24] on a Convex C220 computer (ZDVAM, University of Tubingen) using 6-31G*(” basis sets (**denotes p-functions on hydrogen). For geometry optimizations, standard gradient techniques [25] were applied and the residual forces were below 3 x 10e4 a.u. The influence of electron correlation was taken into account at the MP2 and MP4SDTQ level.

c_3AussIm136

FC(O)NCO

and FC(O)N,

First, the geometric structures for both compounds were optimized at the Hartree-Fock level of theory by going from the planar cis form (S(OCNC) = G(OCNN) = 0’) in steps of 30” to the planar trans conformation (S(OCNC) = G(OCNN) = 180”). In order to study the effect of electron correlation on the relative stabilities and on the conformational behavior due to internal rotation around the C-N single bond, MP2 and

TABLE 5 Relative energies AE (kcalmol-‘) for the cis and tram forms of the pairs FC(O)NCO/HC(O)NCO and barrier heights to internal rotation as calculated by the different theoretical approaches. FC(O)NCO

HC(O)NCO

FC(G)N,

and FC(O)N,/HC(O)N,,

HC(G)N,

0””

90°”

180°”

0”

9o”

180”

0”

90”

180”

0”

90”

180’

Ab Bb Cb Db

0 0 0 0

4.5 3.7 3.8

1.7 0.6 0.6 1.7

0 0 0

14.2 9.5 9.2

3.5 1.3 1.2

0 0 0

4.2 4.1 4.4

2.9 2.0 2.2

0 0 0

14.9 10.6 10.4

5.3 2.6 2.4

ED’ IR’ Ra’

0 0 0

0.8(3) 0.5(2) 0.9(3)

0 0 0

1.3(6) 1.5(4) 1.3(4)

“Values for S(OCNX), X = C (isocyanates) or N (azides); 0” = planar cis, 180” = planar trans, 90’ = barrier to rotation. bA = AE(HF/6-31G”“); B = AE(MP2/6-3 1G”“//HF/6-3 1G”“); C = AE(MP4SDTQ/6-3lG”“//HF/6-3lG*(*‘); D= AE(MP2/6-31G*). ‘AE(=E,,,EC,,) from electron diffraction, IR and Raman spectroscopy (see text).

H.-G. Mack et a/./J. Mol. Strut.,

291 (1993)

207

197-209

1

---FCVl)N,

AEt 10

0 0

kis)

60

120

6 IOCNX)

180

wxls)

Fig. 7. Calculated potential curves (HF/6-31G*) internal rotation around the C-N single bond.

for

7. Vibrational frequencies, IR absorbances (squared dipole moment derivatives) and Raman scattering intensities for the stable forms of FC(O)NCO and FC(0)N3 were obtained at the HF/6-3 1G* level.

in Fig.

HC(O)NCO

and HC(O)N,

For the parent compounds analogous calculations to those for the fluorinated species were performed (see Table 5). Again, the planar cis and trans forms are the only stable structures, with the trans isomers being higher in energy than the cis conformers. The energy differences depend on the theoretical method applied. According to these results, both HC(O)NCO and HC(O)N, should exist in the gas phase only in one conformation. For the barriers to internal rotation the same trends are predicted as for the pair FC(O)NCO/FC(O)N,. Discussion

Electron diffraction analysis of fluorocarbonyl isocyanate results in a mixture of 75(12)% planar cis and 25(12)O/, planar trans. This corresponds to a difference in free energy of AG(ED) = G,,, GciS= 0.7(3) kcal mol-’ . This value agrees, within the experimental error limits, with the AG values

obtained from the IR (AG(IR) = 0.4(2) kcal mol-‘) and Raman (AG(Ra) = 0.8(3) kcalmol-‘) spectra. For comparison between these experimental AG values and the ab initio values for AE (Table 5), the ab initio results (HF/6-31G*) for differences in entropy AS, thermal contributions and zero point vibrational energies were used: AE - AG = 0.1 kcal mol-’ . Thus, the experiments yield AE(ED) = 0.8(3), AE(IR) = 0.5(2) and AE(Ra) = 0.9(3) kcal mol-’ , respectively. The standard ab initio procedures, HF/6-3 lG* and MP2/6-3 lG*, predict an energy difference (1.7 kcal mol-‘) which is too high compared to the experimental value. Good agreement between calculated and experimental AEs, however, is obtained if electron correlation is taken into account (MP2 or MP4SDTQ) with HF/6-3 lG* optimized geometries (Table 5). In the case of FC(0)N3, the electron diffraction experiment yields AG(ED) = G,,,,, G,, = 1.3(6) kcal mol-’ , in very good agreement with AG(IR) = 1.5(4) kcalmol-’ and AG(Ra) = 1.3(4) kcal mol-’ . Ab initio values for AE - AG are less than 0.1 kcal mol- ’ , and were neglected. Comparison with Table 5 shows that, again, the standard ab initio value is too high, and calculations including electron correlation using HF/631G* geometries lead to good agreement with the experiments. Acetyl isocyanate is the only carbonyl isocyanate of the type XC(O)NCO, whose conformational properties were studied in the gas phase, and for which only the cis form is observed [8]. According to our ab initio calculations (Table 5), the parent compound, HC(O)NCO, should also exist only in the cis form at room temperature. Recent HF/6-3 lG* calculations predict the same conformational properties for trifluoroacetyl isocyanate, CF, C(O)NCO, (AE = Etrans- Etis = 2.6 kcal mol-‘) [26]. In the compounds with X = Br, Cl and F, mixtures of planar cis and planar trans forms were found, with the amount of the cis structure increasing in the sequence X = Br (8(5)%) [271, C1(25(8)%) [9] and F (75( 12)%). Obviously there is no correlation between the electronegativity of the substituent X (X = H, CH3, CF,, F, Cl, Br) and the

208

H.-G. Mack et al./J. Mol. Struct., 291 (1993) 197-209

conformational behavior of the corresponding carbonyl isocyanate. FC(0)Nj is the only molecule of the type XC(O)N,, whose gas-phase structure and conformational properties were studied experimentally. The preference of the cis conformation is also concluded from the microwave spectra of methyl azido formate, CH,OC(O)N, [28] and of vinyl azide, H& = CHN, , [ 111.The theoretical results for HC(O)N, (isoelectronic to vinyl azide), as in the case of HC(O)NCO, suggest that only a planar cis form should be observed in the gas phase (Table 5). The structural parameters in the three isocyanates with X = CH3, Cl and F do not differ drastically and have been discussed recently [8]. The most prominent difference in the geometry between FC(O)NCO and FC(O)N, pertains to the C-N =Y Cy = C or N) angle, which is greater by 15” in the isocyanate (125.9(11)0) than in the azide (110.9(9)0). In FC(0)N3 the NNN group deviates more from linearity (170.7(27)0) than the isocyanate moiety in FC(O)NCO (174.6(25)0). In Table 6 structural parameters of some covalent azides are collected and compared with those of fluorocarbonyl azide. In all cases the NB =N, bonds are significantly shorter than the N,=N, bonds. The influence of the electronegativity of the substituents on this bond length difference has been discussed in ref. 23. The experimental bond lengths and angles determined for FC(0)N3 in this work show no major differences compared with the geometries of the azides listed in Table 6.

TABLE

Conclusion

The geometric structures and conformational compositions of fluorocarbonyl isocyanate and azide have been studied by gas-phase electron diffraction, vibrational spectroscopy and ab initio calculations. The experiments result in a predominance of the cis conformer (carbonyl C= 0 bond cis with respect to the N = C or the N =N bonds) for both compounds. The energy differences predicted by ab initio calculations depend on the level of theory applied. Whereas HF/6-31G* and MP2/6-31G* approximations overestimates the energy differences between the two rotamers, MP2/ 6-3 lG*//HF/6-31G* and MP4SDTQ/6-3 lG*//HF/ 6-3 1G* calculations reproduce the experimental values within their error limits. Acknowledgments

H.G.M. gratefully acknowledges financial support by the Deutsche Forschungsgemeinschaft (DFG). C.O.D.V. gratefully thanks Professor Dr. mult. Dr. h.c. Alois Haas (University of Bochum, Germany) for his stimulating contribution and generous support of this work. C.O.D.V. also thanks the Consejo National de Investigaciones Cientificas y Tecnicas (CONICET, Republica Argentina), the Facultad de Ciencias Exactas (Universidad National de La Plata), the Fundacion Antorchas (Republica Argentina) and the DAAD (Germany)

6

Structural parameters of some covalent azides CH,N,’ Bond length (A) X-N N,=N, N, EN,

1.468(S) 1.216(4) 1.130(5)

Bond angle (deg) 116.8(3) XNN 1808 NNN

CF,Nrb

1.425(5) 1.252(5) 1.118(3)

112.4(2) 169.6(34)

“Ref. 29; r, values. bRef. 23; rav values. ‘Ref. pAssumed.

HN,C

1.015(15) 1.243(5) 1.134(2)

108.8(40) 171.3(50)

FN,e

CINAd

1.444(10) 1.253(10) 1.132(10)

1.745(5) 1.252(10) 1.133(10)

108.7(5) 171.9(5)

30; rs values. dRef. 31; t-,/r, values. ‘Ref.

103.8(5) 170.9(10) 32; r,

FC(O)N,

1.390(4) 1.246(S) 1.124( 5)

110.9(8) 170.7(27)

values. fThis work; r, values.

H.-G. Mack et a1.j.I. Mei. Struer., 291 (1993) 197-209

for financial support. H.W. is grateful for a research grant (DFG) and for travel expenses (DAAD and CONICET). References 1 K. Kveseth, R. Seip and D.A. Kohl, Acta Chem. Stand., Ser. A, 34 (1980) 31. L.A. Carreira, J. Chem. Phys., 62 (1976) 3851. Yu. Panch~ko and P. Csaszar, J. Mol. Struct., 130 (1985) 207. 2 K. Kuchitsu, T. Fukuyama and Y. Morino, J. Mol. Struct., 4 (1969) 41. 3 C.E. Blom, G. Grassi and A. Bauder, J. Am. Chem. Sot., 106 (1984) 7427, and references cited therein. 4 K. Hagen, V. Bondybey and K. Hedberg, J. Am. Chem. Sot., 99 (1977) 1365. 5 G. Gundersen, J. Am. Chem. Sot., 97 (1975) 6342. 6 C.H. Chang, A.L. Andreassen and S.H. Bauer, J. Org. Chem., 36 (1971) 920. 7 K. Hagen, V. Bondybey and K. Hedberg, J. Am. Chem. Sot., 100 (1978) 7178. 8 H.-G. Mack, H. Oberhammer and C.O. Della Vcdova, J. Mol. Struct., 265 (1992) 359. 9 H.-G. Mack, H. Oberhammer and CO. Della Vedova, J. Mol. Struct. (Theochem), 200 (1989) 277. 10 A. Bouchy and G. Roussy, J. Mol. Spectrosc., 68 (1977) 156. C. Kirby and H.W. Kroto, J. Mol. Spectrosc., 70 (1978) 216. 11 R.G. Ford, J. Mol. Spectrosc., 65 (1977) 273. 12 CO. Della Vedova, Z. Anorg. Allg. Chem., 609 (1992) 150. 13 H. Willner, in preparation. 14 H. Oberhammer, Molecular Structure by Diffraction Methods, Vol. 4, The Chemical Society, London, 1976, p. 24.

209 15 H. Oberhammer, W. Gombler and H. Willner, J. Mol. Struct., 70 (1981) 273. 16 W.A. Seth-Paul, J. Mol. Struct., 3 (1969) 403. 17 S. Mizushima, T. Shimanouchi, J. Nakagawa and A. Miyake, J. Chem. Phys., 21 (1953) 215. 18 A.H. Nielsen, T.G. Burke, P.J.H. Woltz and E. Jones, J. Chem. Phys., 20 (1952) 596. 19 K. Gholivand, G. Schatte and H. Willner, Inorg. Chem., 26 (1987) 2137. 20 J. Haase, Z. Naturforsch., Teil A, 25 (1970) 936. 21 J. Boggs, in A. Domenicano and I. Hargittai (Eds.), Accurate Molecular Structures, Oxford University Press, 1992, p. 322. 22 H.-G. Mack, CO. Della Vedova and H. Oberhammer, Angew. Chem., 103 (1991) 1166; Angew. Chem., Int. Ed. Engl., 30 (1991) 1145. 23 K.O. Christe, D. Christen, H. Oberhammer and C.J. Schack, Inorg. Chem., 23 (1984) 4283. 24 GAUSSIAN~~, M.J. Frisch, J.S. Binkiey, H.B. Schlegel, K. Ra~avachari, C.F. Melius, R.L. Martin, J.J.P. Stewart, F.W. Bobrowicz, C.M. Rohlfing, R.L. Kahn, D.J. DeFrees, R. Seeger, R.A. Whiteside, D.J. Fox, E.M. Fleuder and J.A. Pople, Carnegie-Mellon Quantum Chemistry Publishing Unit, Pittsburgh, PA, 1986. 25 P. Pulay, Theor. Chim. Acta, 50 (1979) 299. 26 H.-G. Mack, unpublished results, 1990. 27 C.O. Della Vedova, Ph.D. Thesis, University of Bochum, 1990. 28 R.K. Kakar, C.R. Quode, W. Lowowski and R.E. Wilde, J. Chem. Phys., 72 (1980) 4123. 29 D.W .W. Anderson, D.W.H. Rankin and A. Robertson, J. Mol. Struct., 14 (1972) 385. 30 B.P. Winnewisser, J. Mol. Spectrosc., 82 (1980) 220. 31 R.L. Cook and M.C.L. Gerry, J. Chem. Phys., 53 (1970) 2525. 32 D. Christen, H.-G. Mack, G. Schatte and H. Willner, J. Am. Chem. Sot., 110 (1988) 707.