Normal coordinate analyses, vibrational assignments and barrier to internal rotation in isocyanatoacetaldehyde based on ab initio calculations

Journal of Molecular Structure (Theochem) 454 (1998) 41–50

Normal coordinate analyses, vibrational assignments and barrier to internal rotation in isocyanatoacetaldehyde based on ab initio calculations Wolfgang Fo¨rner, Hassan M. Badawi * Department of Chemistry, King Fahd University of Petroleum and Minerals, Dhahran 31261, Saudi Arabia Received 30 March 1998; accepted 20 April 1998

Abstract The structure and conformational stability of isocyanatoacetaldehyde were investigated using ab initio calculations. The calculations were carried out at RHF/6-311G* and MP2/6-311G* levels. From the calculation the molecule was predicted to exist predominantly in the cis–cis conformation. The potential function for the internal rotation of the CHO group was determined for the molecule. The inclusion of electron correlation into the calculations had a very small effect on the calculated potential coefficients. The vibrational frequencies were computed at the Hartree–Fock level. Normal coordinate calculations were carried out and potential energy distributions were calculated for the cis–cis conformer of the molecule. The calculated vibrational frequencies for the two conformers were scaled and compared to those observed experimentally for similar molecules. 䉷 1998 Elsevier Science B.V. All rights reserved. Keywords: Ab initio; Rotational barrier; Normal coordinate analyses; Vibrational assignment; Isocyanatoacetaldehyde

1. Introduction Recently, the structure and conformational stability of a series of organic compounds of the general formula R–CH 2 –CXO, where, X is H, F, and Cl, with R being cyanide (NxC–) and acetylinic (H–CxC–) groups were investigated by ab initio calculations [1,2]. The objective of these studies was to study the nature of the interaction between the carbonyl moiety and the bulky substituents. The chemistry and reactivity of the corresponding isocyanato derivatives (R is the OyCyN– group) are of great importance for heterocyclic organic synthesis [3,4]. In these compounds there are two internal rotors that control * Corresponding author.

their conformational preference, the carbonyl and the R–CH 2 – groups. The rotation of these two rotors around C–C bonds results in a complex conformational equilibrium. Therefore, as a continuation of our studies, we report in the present study the investigation of the conformational behavior of isocyanatoacetaldehyde for the purpose of comparison. In this work, ab initio optimization of the energies was carried out for all the stable conformers and the transition states of the molecule. From the data the relative conformational stability and the barrier to internal rotation were determined. Additionally, vibrational frequencies were calculated and a tentative assignment was made for all the normal modes by using normal coordinate calculations. The results of the work are presented herein.

0166-1280/98/$ - see front matter 䉷 1998 Elsevier Science B.V. All rights reserved. PII: S 01 66 - 12 8 0( 9 8) 0 02 1 1- 5

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W. Fo¨rner, H.M. Badawi / Journal of Molecular Structure (Theochem) 454 (1998) 41–50 Table 2 ˚ and degrees), total dipole moment Structural parameters (A (Debye), and rotational constants (MHz) of the cis–cis conformation of isocyanatocarboxaldehyde Parameter

HF/6-311G* Cis–cis

Fig. 1. Atom numbering for isocyanatoactaldehyde in trans–cis (upper) and trans–trans (lower) conformations.

2. Ab initio calculations The Gaussian 94 program [5], running on an IBM RS/6000 model 7015-R24 workstation, was used to carry out the LCAO-MO-SCF restricted Hartree– Fock (HF) calculations. The calculations were carried out at HF/6-311G* and MP2/6-311G* levels. The structures of isocyanatocarboxaldehyde in its possible stable conformations were optimized. The energies, rotational constants and dipole moments were predicted. From preliminary calculations, the gauche conformers were found to be of considerably higher energy than the planar cis–cis conformer as shown in Figs. 2 and 3. The structural parameters of the molecule in its stable conformers (Fig. 1) were optimized by minimizing the energy with respect to all the geometrical parameters. The conformational stabilities were determined by comparing the calculated total energies (Table 1). The calculated structural parameters are listed in Table 2 and compared to those

Bond length 1.511 r(C 1 –C 2) r(C 1 –N) 1.426 1.178 r(C 2yO) 1.096 r(C 2 –H) 1.189 r(C 3yN) r(C 3yO) 1.142 1.086 r(C 1 –H 1) 1.086 r(C 1 –H 2) Bond angle 114.4 (C 2C 1N) 124.1 (C 1C 2O) 114.6 (C 1C 2H) 136.1 (C 1NC 3) 172.7 (OC 3N) 108.2 (C 2C 1H 1) (C 2C 1H 2) 108.2 106.2 (H 1C 1H 2) 122.7 (H 1C 1C 2N) −122.7 (H 2C 1C 2N) (NC 1C 2O) 0.0 Dipole moment 4.8 (m t) Rotational constants A 7365 B 2930 C 2123 a

MP2/6-311G* Electron diffraction Cis–cis

Cis

1.518 1.432 1.210 1.108 1.222 1.175 1.097 1.097

1.388 1.192 1.215 1.154

114.2 123.9 114.8 132.0 169.7 108.1 108.1 106.2 122.7 −122.7 0.0 5.0

123.5 111.6 125.9 174.6

122.7 −122.7 0.0

7030 2989 2125

Data are obtained for fluorocarbonyl isocyanate [6].

Table 1 Calculated total energies (Hartrees) and relative energies (K cal mol −1) of stable conformers of isocyanatoacealdehyde Conformation

Cis–cis Cis–trans Trans–cis Trans–trans a

(v, f)

a

(0, 0) (0, 180) (180, 0) (180, 180)

HF/6-311G*

MP2/6-311G*

Total energy

Relative energy

Total energy

Relative energy

−319.59232 −319.58807 −319.58715 −319.59054

0.000 2.665 3.244 1.112

−320.53459 −320.53019 −320.52915 −320.53122

0.000 2.760 3.413 2.111

v and f are NCO and CHO torsional angles respectively.

a

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Fig. 2. Potential curves for the CHO torsion in isocyanatoacetaldehyde as determined by ab initio calculations with the HF/6-311G* (———), and MP2/6-311G* (- - -) levels.

obtained from electron diffraction data for fluorocarbonyl isocyanate [6]. 2.1. Potential surface 2.1.1. CHO torsional potential function The potential surface scan for the internal rotation about the C–C single bond was obtained by allowing OCCN dihedral angle (f) to vary by 10⬚ increments from 0⬚ (cis position) to 180⬚ (trans position). All the remaining parameters were held constant at the optimized parameters for the cis conformer (OCCN dihedral angle v is 0⬚). The saddle points were determined and full geometry optimization was then carried out at the transition states. Additionally, full geometry optimization at each of the fixed OCCN dihedral angles (f), 15⬚, 30⬚, 45⬚, 75⬚, 90⬚, 105⬚, 135⬚, 150⬚ and 165⬚,

were carried out at various levels of calculations. The torsional potential was represented as a Fourier cosine series in the dihedral angle (f): V(f) = S n(V n/2)[1 − cos(nf)], where the potential coefficients V 1 –V 6 are considered adequate to describe the potential function. The results of the energy optimizations were used to calculate the six coefficients by leastsquares fitting. The data are listed in Table 3 the potential function for CHO torsion in the molecule is shown in Fig. 2. 2.1.2. NCO torsional potential function The potential function for the NCO internal rotation about the C–C single bond was obtained by varying the CNCC dihedral angle (v) by 10⬚. Single point optimizations at optimized structural parameters of the cis–cis conformer (CNCC is 0⬚) were carried

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Table 3 Calculated potential constants (K cal mol −1) for the internal rotation of the CHO rotor in isocyanatoactealdehyde Potential constants

HF/6-311G*

MP2/6-311G*

V1 V2 V3 V4 V5 V6

0.692 4.265 1.962 0.249 0.011 0.008

0.771 4.043 1.977 0.341 0.012 0.003

out at the two levels of calculations. The NCO potential curve for the molecule is shown in Fig. 3. 2.1.3. Vibrational frequencies and normal coordinate analyses The vibrational frequencies for the cis–cis

conformer of isocyanatoacetaldehyde were computed the at HF/6-311G* level. The molecule in this conformation has C s symmetry. The 21 vibrational modes span the irreducible representations: 14 A⬘ and 7 A⬙. The A⬘ modes should be polarized while the A⬙ modes should be depolarized in the Raman spectra of the liquid. Normal coordinate analyses were carried out for the stable conformer of the molecule in order to provide a complete assignment of the fundamental vibrational frequencies. A computer program was written for this purpose following Wilson’s method [7]. The cartesian coordinates for the stable conformers together with the normal modes (in cartesian coordinates) and the frequencies from the Gaussian 94 output were used as input in the program. A complete set of internal coordinates (Table 4) was used to form symmetry

Fig. 3. Potential curves for the NCO torsion in isocyanatoacetaldehyde as determined by ab initio calculations with the HF/6-311G* (———), and MP2/6-311G* (- - -) levels.

W. Fo¨rner, H.M. Badawi / Journal of Molecular Structure (Theochem) 454 (1998) 41–50 Table 4 Internal coordinate definitions a for isocyanatoacetaldehyde No.

Coordinate

Definition

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22

C 1 –C 2 stretch C 1 –N 3 stretch C 2 –O 4 stretch C 2 –H 5 stretch N 3 –C 6 stretch C 6 –O 7 stretch C 1 –H 8 stretch C 1 –H 9 stretch C 2C 1N 3 bend C 1C 2O 4 bend H 5C 2O 4 bend C 1C 2H 5 bend C 6N 3C 1 bend O 7C 6N 3 bend C 2C 1H 8 bend N 3C 1H 8 bend H 8C 1H 9 bend C 2C 1H 9 bend N 3C 1H 9 bend CHO wag NCO wag (C 2C 1N 3C 6 C 6N 3C 1H 8 C 6N 3C 1H 9) torsion (N 3C 1C 2O 4 N 3C 2C 1H 5 O 4C 2C 1H 8 H 5C 2C 1H 8 O 4C 2C 1H 9 H 5C 2C 1H 9) torsion

R S P T Q Z A D b e v f j p g r d a k q x

23

a

t1

t2

For atom denotation see Figure 1.

coordinates (Table 5). We could not separate the two CHO and NCO asymmetric torsions and assumed that the two modes couple strongly with each other. Therefore, the corresponding symmetry coordinates were defined as a combination of the two coordinates as shown in Table 5. The normal modes were transformed to mass-weighted cartesian coordinates, which were then used to calculate the force constant matrix. This was diagonalized and its eigenvectors and eigenvalues were used in the further calculations. In the output of Gaussian 94 the normal modes are given to an accuracy of only two digits after the decimal points. Therefore, the frequencies obtained in this way are expected to differ from those obtained from the SCF calculation by at most 1 cm −1 with the larger differences at higher frequencies (less than 1%

45

error). Following this step the force constant matrix was transformed to internal coordinates. To ensure correctness, this transformation was checked numerically in both directions. At this point, the force constant matrix in internal coordinates could be scaled, if desired, back-transformed to mass-weighted cartesians and diagonalized again to obtain scaled frequencies and normal modes. The matrix was finally transformed to symmetry coordinates where, again, all possible numerical checks were performed. In the next step the normal modes were also transformed to symmetry coordinates, together with the force constant matrix in symmetry coordinates and the frequencies from the diagonalization. Finally, the potential energy distribution (PED) for each normal mode among the symmetry coordinates was calculated and is given in Tables 4 and 5. A tentative assignment of the fundamentals was proposed. The assignments were made based on calculated PED, infrared band intensities, Raman line activities and depolarization ratios and on those reported for similar molecules [6,8–10]. The data of the vibrational assignments are listed in Table 6. 2.1.4. Calculation of vibrational spectra For the calculation of Raman spectra we use the scattering activities S j, the wavenumbers n j and the depolarization ratio r j for each normal mode as calculated in the RHF/6-311G* run for our molecule. Then the Raman cross-sections (⳵j j/⳵Q), which are proportional to the intensities, are given as [6,13]: ⳵jj =⳵Q = (24 p4 =45)(no − nj )4 (h=8p2 cnj ) × Sj [(1 − rj )=(1 + rj )]=[1 − exp( − hcnj =kB T)] Since we use only relative intensities, we calculated them as: Ij = (⳵jj =⳵Q)=(⳵jjm =⳵Q) where the subscript ‘jm’ denotes the normal mode having the largest Raman cross-section. As laser wavelength we took l o = 514.5 nm (n o = 1/l o), which corresponds to an argon ion laser. We assumed the temperature to be T = 300 K. Then the line shapes are calculated as Lorentzians with a width Dn = 1 cm −1. Thus, the final spectrum is

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W. Fo¨rner, H.M. Badawi / Journal of Molecular Structure (Theochem) 454 (1998) 41–50

Table 5 Symmetry coordinates for isocyanatoacetaldehyde Species

Description

A⬘

CH 2 C–H ald NCO C 2yO 4 NCO CH 2 CH ald CH 2 C 1 –N 3 C 1 –C 2 CCO NCO NCC CNC CH 2 CH 2 CH ald CH 2 NCO Asymmetric torsion I Asymmetric torsion II

A⬙

a

Symmetry coordinate symmetric stretch stretch antisymmetric stretch stretch symmetric stretch scissor in-plane bend wag stretch stretch in-plane bend in-plane bend in-plane bend in-plane bend antisymmetric stretch twist out-of-plane bend rock out-of-plane bend

S1 = A + D S2 = T S3 = Q − Z S4 = P S5 = Q + Z S 6 = [(6) 1/2 + 2]d − [(6) 1/2 − 2]b − g-a-r-k S7 = f − v S8 = g + a − r − k S9 = S S 10 = R S 11 = f + v + 2e S 12 = p S 13 = [(6) 1/2 − 2]d − [(6) 1/2 + 2]b + g + a + r + k S 14 = j S 15 = A + D S 16 = g − a − r + k S 17 = q S 18 = g − a + r − k S 19 = x S 20 = t 1 + t 2 S 21 = t 1 − t 2

Not normalized.

calculated as: I(n) = ∑ Ij L(n − nj ) j

L(n − nj ) = (1=p)(Dn=2)=[(n − nj )2 + (Dn=2)2 ] …

a

L(n) dn = 1 where the integration must be performed from −⬁ to +⬁ and j runs over all normal modes. For the plots we used a grid of step size 5 cm −1, but not when a spectral line appears between two consecutive grid points. In this case, we inserted 20 points with a step size of 0.5 cm −1 into this interval which includes the exact location of the center of the line. All frequencies obtained by the HF calculation were scaled with a common factor of 0.9 prior to the intensity calculation. The calculated Raman spectrum is presented in Fig. 4. For the infrared spectrum we used the intensities as given by the HF calculation (relative to the largest one) and converted them to relative transmittance by subtracting them from 1. The infrared spectrum is shown in Fig. 5.

3. Discussion The conformational behavior of cyanoacetaldehyde and cyanoacetyl fluoride and chloride [1], and their acetylinic analogue [2], were found to be similar to that of haloacetaldehyde and haloacetyl halides [6,11,12]. For the aldehydes, the planar trans conformation with the carbonyl oxygen directed away from the substituent was determined to be the lowest energy form. The replacement of the aldehydic hydrogen with the halogen atoms significantly influences the conformational behavior of the corresponding halides. In the case of these halides, there are two forces that control the conformational behavior of the molecules. The first is the net columbic interactions and the second is the steric hinderance between the substituent and the carbonyl moiety. In the fluorides, the repulsive force between the carbonyl halogen and the substituent stabilizes the molecules in the cis conformation with the trans being the higher energy conformer. For the chlorides, the gauche and not the trans form was found to be thermodynamically the second stable energy form [1,2].

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Table 6 Calculated vibrational frequencies a (cm −1) at the HF/6-311G* level for the cis–cis conformer of isocyanatoacetaldehyde Symmmetry Number A⬘

A"

c

Frequency Scaled

IR int

Raman active

Depol. ratio

Exp.

n1 n2 n3 n4 n5

3209 3144 2498 2022 1645

2888 2830 2248 1820 1481

51.4 90.6 1646.6 117.9 10.9

170.49 140.6 0.9 6.7 6.7

0.0 0.4 0.1 0.6 0.1

2906(2989) 2831 2285(2270) 1752 1393(1480)

n6 n7 n8 n9

1607 1528 1516 1055

1446 1375 1364 950

19.1 36.9 11.9 3.5

19.2 8.3 10.4 6.4

0.5 0.6 0.4 0.6

1480(1453) 1378 1352(1312) 807(901)

n 10 n 11

905 833

815 750

38.5 120.7

8.4 2.9

0.1 0.4

n 12

703

633

6.8

0.3

0.7

n 13

331

298

27.0

1.6

0.7

n 14

121

109

2.8

0.6

0.7

144(175)

n 15 n 16 n 17 n 18 n 19 n 20 n 21

3233 1387 1206 798 701 209 63

2910 1248 1085 718 631 209 63

19.0 0.0 0.3 0.4 49.8 1.6 19.8

86.3 5.5 2.9 5.4 0.3 1.6 0.0

0.8 0.8 0.8 0.8 0.8 0.8 0.8

2956(3013) 1258(1223) 1020 788(959) 610(578)

640(655)

PED

b

97% CH 2 sym. stretch 97% CH ald stretch 98% NyCyO antisym. stretch 92% CyO stretch 61% NyCyO sym. str., 21% C–N str., 14% CH 2 scissor 83% CH 2 scissor 83% CH ald bend 77% CH 2 wag, 12% C–C str. 30% C–N str., 23% C–C str., 20% NyCyO sym. str., 11% CH 2 wag, 11% N–C–C bend 59% C–C str. (S 10), 18% C–N str. 28% C–CyO bend, 29% N–C–C bend, 28% NyCyO bend, 12% C–N str. 63% NyCyO bend, 18% C–CyO bend, 13% C–N str. 41% N–C–C bend, 31% C–CyO bend, 18% CyN–C bend 81% CyN–C bend, 16% N–C–C bend

100% CH 2 antisym. str. 92% CH 2 twist 55% CH ald bend, 40% CH 2 rock 60% CH 2 rock, 32% CH ald bend 97% NyCyO bend 49% torsion II, 45% torsion I 53% torsion I, 47% CHO torsion II

a Scaled frequencies are obtained with factors of 0.9 for stretches and bending modes, and 1.0 for the torsions. IR intensities and Raman ˚ 4 amu −1 respectively. activities are calculated in K m mol −1 and A b Proposed vibrational assignment is denoted as italic. c Experimental frequencies cm −1 are obtained from Refs. [10–12] for chloromethyl isocynate (values in parentheses), ethyl isocyanate, and chloroacetaldehyde (values are denoted as italic) respectively.

In the case of isocyanatocarboxaldehyde, the cis– cis conformer was predicted to be predominantly the preferred form. The pronounced repulsive interaction between the carbonyl oxygen and the electron lonepair on the nitrogen was the main force that determines its relative conformational stability. This force considerably destabilizes the molecule in the conformations with the isocyanato (–NyCyO) group being gauche or trans with respect to the carbonyl moiety. Therefore, all the gauche conformations were predicted to be of relatively much higher energy than the planar cis–cis conformer.

The calculated structural parameters of the lowest energy conformer were compared to those obtained from electron diffraction data for fluorocarbonyl isocyanate [6]. As shown in Table 1, the inclusion of electron correlation in the calculation made a small change in the relative energies of the molecule. However, some structural parameters were noticed to be influenced by electron correlation. For example, the aldehydic C–H, C–N, CyO and NyO bond distances are calculated to be longer at the MP2 level. The calculated bond angles are generally consistent with the reported angles within the expected experimental uncertainties.

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W. Fo¨rner, H.M. Badawi / Journal of Molecular Structure (Theochem) 454 (1998) 41–50

Fig. 4. Calculated vibrational Raman spectrum of the cis–cis isocyanatoacetaldehyde at the RHF/6-311G* level. All calculated frequencies are scaled by a factor of 0.9.

We have performed the normal coordinate calculations to provide complete vibrational assignment for the normal modes of isocyanatoacetaldehyde. From these calculations we calculated the potential energy distribution for each normal mode among the symmetry coordinates for the molecule in the cis–cis conformation. The vibrational assignments of most of the fundamental vibrations were straightforward, based on the calculated PED. Some of the calculated modes were predicted to be highly mixed with other modes. However, their assignments were made possible by a direct comparison with those made from experimental data in similar molecules [6,9–12]. For the fundamentals that are associated with C–H modes, there are three stretches. These stretches were calculated to have the highest Raman activities in the Raman spectra of the molecule (Table 6). The aldehydic C–H stretch was predicted to be lower in

frequency than the two CH 2 stretches. The two in- and out-of-plane bending modes for the aldehydic C–H were calculated at 1375 cm −1 and 1085 cm −1. These values are consistent with the observed frequencies at 1378 cm −1 and 1020 cm −1 in the infrared spectrum of the gaseous chloroacetaldehyde [12] respectively. The four CH 2 bending modes were calculated to be in the order: scissor ⬎ wagging ⬎ twisting ⬎ rocking modes, as one might expect. This agrees very well with the assignments made for the corresponding observed modes in chloroacetaldehyde [12]. The CyO stretching mode was calculated to have a very small degree of mixing (PED of 92%). The assignment of this fundamental vibration was very straight to n 5 in Table 6. The corresponding in-plane CCO bending mode was predicted to be highly mixed with other modes (PED of 28% CCO bend, 29% NCC bend, and 28% NCO bend). From the PED values n 12 (61%) and n 13 (41%) were clearly assigned to the

W. Fo¨rner, H.M. Badawi / Journal of Molecular Structure (Theochem) 454 (1998) 41–50

NCO and NCC bending modes respectively. Therefore, n 11 can be assigned to CCO bend. For the NCO group, the mode with the highest infrared intensity was calculated to have a PED of 98% antisymmetric stretch. This agrees with experimental observation in the infrared spectra of fluorocarbonyl isocyanate [6], chlorocarbonyl isocyanate [9], chloromethyl isocyanate [10], and ethyl isocyanate [11]. On the other hand, the symmetric –NyCyO stretch was assigned to the scaled line at 1481 cm −1 (PED of 61%). The scaled vibration at 950 cm −1 was calculated to a PED of 30% C–N stretch, 23% C–C stretch, 20% NyCyO symmetric stretch, 11% CH 2 wag, and 11% N–C–C bend. The NyCyO symmetric stretch, the CH 2 wag, the C–C stretch, and the N–C– C bend were confidently assigned to n 5, n 8, n 10, and n 13 modes respectively. Furthermore, in the infrared spectrum of the gaseous chloromethyl isocyanate, the band

49

at 901 cm −1 was assigned to C–N stretch [10]. Therefore, we assigned n 9 to this vibrational mode. On comparison, many of the calculated bending vibrations that are associated with the NCO group agree reasonably well with those observed in the vibrational spectra of similar molecules [6,9–12]. For example, the in- and the out-of-plane –NyCyO bendings were calculated and scaled at 633 cm −1 and 631 cm −1, which agree with those observed for ethyl isocyanate at 640 cm −1 and 610 cm −1 respectively [11]. The lowest A⬙ modes in the spectrum of isocyanatoacetaldehyde are the CHO (n 20) and the NCO (n 21) asymmetric torsions. These two modes are calculated to be at 209 cm −1 and 63 cm −1 with a predicted IR intensity of 1.6 and 19.8 respectively (see Fig. 5). As mentioned earlier we could not separate the two torsions and defined their symmetry coordinates as a combination of the two modes. On the basis of the

Fig. 5. Calculated vibrational infrared spectrum of the cis–cis isocyanatoacetaldehyde at the RHF/6-311G* level. All calculated frequencies are scaled by a factor of 0.9.

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calculated PED, clearly these two vibrations highly mix with each other. This means that the two torsions extensively couple with each other.

Acknowledgements The authors gratefully acknowledge the support of this work by King Fahd University of Petroleum and Minerals.

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