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The electronic structure of some acyl azides: cyclic–open tautomerism
18 February 2000
Chemical Physics Letters 318 Ž2000. 276–288 www.elsevier.nlrlocatercplett
The electronic structure of some acyl azides: cyclic–open tautomerism Rafie H. Abu-Eittah ) , Hussein Moustafa, Ahmad M. Al-Omar Department of Chemistry, Faculty of Science, Cairo UniÕersity, Giza, Egypt Received 20 January 1999; in final form 12 November 1999
Abstract Ab initio molecular orbital calculations including correlation interaction have been performed on formoyl, thioformoyl and acetyl azides using fairly large basis sets. The structural and energetic parameters were calculated at optimized geometry. Cyclic–open tautomerism has been theoretically investigated and the results showed that formoyl azide and acetyl azide are more stable than the corresponding cyclic conformers, oxatriazoles, whereas 1,2,3,4-thiatriazole is more stable than thioformoyl azide. The process of cyclic open conversion of the studied compounds was investigated theoretically and the energy of activation of such process was calculated. q 2000 Elsevier Science B.V. All rights reserved.
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1. Introduction The reviews by Evans et al. w1x and by Gray w2x give very valuable surveys on the physical properties and thermodynamic and decomposition processes of inorganic azides. Bonding in nitrogen compounds w3x and in inorganic azides w4x was discussed. The electronic structures as well as molecular orbital ŽMO. calculations were performed on some benzene azides w5x. Most of the reported studies are not up to date and accurate theoretical work on the electronic structure and properties of azides is still missing. As for carbonyl azide, the situation is less satisfactory. In fact, the first ultraviolet ŽUV. spectrum of
) Corresponding author. Fax: q20-202-5727556; e-mail: [email protected]
benzoyl azide is reported in 1966 by Bhaskar w6x. Attachment of a aC5O group to an azide group
will affect the geometry, reactivity and structure of the azide group and hence the acyl azide molecule. This is clear when comparing the dipole moment of C 6 H 5 N3 which is 1.44 D w7x, using the benzene as a solvent, with that of C 6 H 5 CON3 which is 2.60 D w7x in the same solvent. Along the same lines, the dipole moment of CH 3 COCH 2 N3 is 3.64 D w8x in benzene as a solvent whereas that of CH 2 5CH–CH 2 –N3 is 1.92 D w7x. The first members of acyl azides, HCON3 , CH 3 CON3 , C 2 H 5 CON3 , or their cyclic conformers, 1,2,3,4-oxatriazole, 5-methyl- or 5-ethyl-1,2,3,4oxatriazole, have not yet been prepared. In this work, a theoretical treatment on a non-empirical basis has
0009-2614r00r$ - see front matter q 2000 Elsevier Science B.V. All rights reserved. PII: S 0 0 0 9 - 2 6 1 4 Ž 9 9 . 0 1 3 3 1 - 7
R.H. Abu-Eittah et al.r Chemical Physics Letters 318 (2000) 276–288
been carried out to investigate the conformations, geometry and electronic structure of these molecules. MO calculations using the ab initio method and different basis sets of which are 6-31G ) , 6-311G, 6-311G ) , DZ, DZ ) , TZ, TZ ) and TZ ) ) were tried. As the basis set is expanded, including d-functions as in the starred basis sets and both d- and p-functions as in the double starred sets, the total energy is lowered. To account for electron correlation, MP methods were used. MO calculations were performed using both HF and MP methods on the optimized geometry of the ground state of both the cyclic and open conformers of the studied molecules. The process of cyclic open conversion was investigated theoretically by MO calculations using HFr6-31G method along the reaction coordinate.
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277
The HF energy of the molecule is nr2
E HF s 2
Ý
nr2 nr2
Hicore q i
is1
Ý Ý Ž 2 Ji j y K i j . q VNN , is1 js1
Ž 6. core < Hˆicore ' ² f i Ž 1 . < HˆŽ1. fi Ž 1. : i
¦
Za
' f i Ž 1 . y 12 =12 y Ý
r1 a
a
;
fi Ž 1. ,
Ž 7.
and
¦
Ji j s f i Ž 1 . f j Ž 2 .
1 r 12
;
fi Ž 1. f j Ž 2. .
Ž 8.
The molecular orbitals f i are normalized and the closed-subshell orthogonal HF MOs satisfy Fˆ Ž 1 . f i Ž 1 . s ´ i f i Ž 1 . ,
Ž 9.
2. Computational methods
where ´ i is the orbital energy and the Fock operator Fˆ is given by
2.1. Electronic states
core Fˆ Ž 1 . s HˆŽ1. q
nr2
For a closed-shell configuration containing n electrons, the molecular Hartree–Fock ŽHF. wavefunction is written as an antisymmetrized product of spin orbitals. The HF molecular energy E HF is w9x: EHF s ² D < Hˆel q VNN < D : ,
Ž 1.
where D is the normalized Slater-determinant HF wavefunction, D s < f 1 . . . f 1 fn fn <
Ž 2.
and ² D < VNN < D : s VNN ² D < D : s V NN .
Ž 3.
V NN is the potential energy due to nuclear repulsion. The electronic Hamiltonian Hˆel is the sum of oneelectron operator, Fˆi , and two-electron operators, gˆ i j Hˆel s Ý Fˆi q Ý i
j
Ý gˆ i j ,
Ž 4.
j)i
where 1 Za Fˆi s y =i2 y Ý 2 a ri a
Ý
2 Jˆj Ž 1 . y Kˆ j Ž 1 . ,
Ž 10 .
js1
and
gˆ i j s
1 ri j
.
Ž 5.
´ i s f i) Ž 1 . Fˆ Ž 1 . f i Ž 1 . dÕ 1 .
H
Ž 11 .
Substitution and summing over the nr2 occupied orbitals gives nr2
E HF s 2
nr2 nr2
Ý ´i y
Ý Ý Ž2 J
is1
is1 js1
ij
yK i j . q VNN .
Ž 12 .
MO calculations were carried out using the GAMESS program, version 1998, distributed free by the authors of the program w10x. Large numbers of basis sets were attempted, the best being that which leads to a minimum total energy of the ground state. One starts with the minimal STO and continues using the split-valence DZ and TZ basis sets which adds porbitals to the H-atom and d-orbitals to atoms other than the H-atom. Addition of d-orbitals affected the value of the energy significantly whereas addition of p-orbitals has a minor effect. Calculations were carried out on a PC Pentium 300 MHz under Windows 95. Actual molecular applications of MP perturbation theory began only in 1975 with the work of Pople and co-workers w11x and Bartlette and co-workers w12x.
278
R.H. Abu-Eittah et al.r Chemical Physics Letters 318 (2000) 276–288
The MP unperturbed Hamiltonian H o is taken as the sum of the one-electron Fock operator, and the ground-state HF wavefunction Fo is one of the zero-order wavefunctions. The perturbation H X is the difference between the true molecular electronic Hamiltonian H and H o . Formulas for the MP energy correction E Ž2., E Ž3., E Ž4., et cetera, have been derived w11x. In MP4 calculations the terms that involve triply substituted determinants is very time consuming and hence are sometimes neglected. A commonly used approximation is designated by MP4-SDQ where SDQ Žsingle, double and quadruple. excitations are involved. 2.2. Basis sets The success of MO calculation depends on the use of an adequate basis set. In the LCAO method each MO is expressed as a finite sum of Slater-type orbitals ŽSTO. or Gaussian-type functions ŽGTF.. Instead of using individual Gaussian functions as basis functions, contracted GTFs, each of which is a linear combination of a number of primitive Gaussian functions, are used. A considerable increase in computational efficiency can be obtained if the components of Gaussian primitives are shared between different basis functions w13x. At the split-valence level this has been exploited by sharing primitive exponents between s- and p-functions for the valence functions. A series of basis sets has been defined and designated K-LMG where K, L and M are integers. Such a basis set for a first-row element consists of an s-type inner-shell function with K Gaussians, an inner-set of valence s- and p-type functions with L Gaussians, and another outer sp set with M Gaussians. Both valence sets have shared exponents. Some such split-valence sets were developed w14,15x and denoted 4-31G, 5-31G and 6-31G. In each case, the valence of a first-row atom is represented by an inner three-Gaussian function and outer single-Gaussian function. The inner shell Žexpect for H-atom. is represented by a single function which is the sum of four, five or six Gaussian functions. Addition of the polarization function gives additional flexibility to the description of molecular orbitals w16x. These polarization functions are second-order Gaussians for non-H-atoms and first-order Žp-type Gaussians. for hydrogen. Such functions have been added to the
6-31G split-valence basis to give the sets 6-31G ) Žpolarization function only on heavy atoms. and 6-31G ) ) Žpolarization functions on heavy and Hatoms.. A contracted Gaussian basis set Ž6-311G ) ) . has been developed by optimizing exponents and coefficients at the Møller–Plesset ŽMP. second-order level for the ground states of first-row atoms w17x. This has a triple splitŽ311. in the s- and p-valence shells associated with a six-Gaussian inner shell together with a single set of uncontracted polarization functions on each atom. This basis set partly overcomes some of the shortcomings of 6-31G, 6-31G ) and 6-31G ) ) basis set. The double-zeta ŽDZ. basis sets were introduced to improve energy calculation w18x. In the DZ basis set, the number of contracted Gaussian-type orbitals ŽCGTO. is twice the number of occupied atomic orbitals ŽAO. that differ in their exponents Ž z .. The DZ basis set for H-atom is Ž3s.rw2sx and for C-, Oand N-atoms is Ž9s, 5p.rw3s, 2px. The triple-zeta ŽTZ. basis set was introduced to describe each AO as a combination of three STOs which differ in their exponents w19x. For H-atom the TZ basis set is Ž5s.rw3sx and for C-, N- and O-atoms is represented by Ž10s, 6p.rw5s, 3px. For heavy atoms such as sulfur ŽS. the TZ basis set is represented by Ž13s, 10p.rw6s, 5px.
3. Result and discussions 3.1. Formoyl azide 3.1.1. Open conformer Although carbamoyl azide
has been known since 1894 w20–22x and many of its derivatives are also known, yet compounds of the general formula
R.H. Abu-Eittah et al.r Chemical Physics Letters 318 (2000) 276–288
where R s H, alkyl or phenyl are scarcely studied in the literature. Such compounds are, generally, not easy to prepare and in many cases are unstable. Formoyl azide is a gas and has not yet been prepared. None of its physical properties or spectral data are known. The cyclic tautomer, oxatriazole, has not been reported. In this work, one investigates theoretically electronic structure, geometry and energies of such groups of compounds.
279
tried, starting with a single minimal basis set STO-6G and ending with a sophisticated one – TZ ) ) . The best basis set, TZ ) ) , gave rise to y276.67049 a.u. for the total energy of HCON3 whereas the STO-6G basis set gave rise to y275.56434 a.u. To account for electron correlations, perturbation methods are used. Møller–Plesset ŽMP. perturbation method were used and the total E, equilibrium geometry and dipole moment were calculated for formoyl azide. The total energy decreased to y272.97797 and y272.97222 a.u. when MP2 and MP4-SDQ methods were used. The calculated electron correlation energy is y0.56356 a.u. The calculated energetic parameters are given in Table 1 whereas the geometrical parameters are given in Table 2. The geometrical variables calculated using the TZ ) ) basis set are comparable with those calculated using the MP method and both are expected to be near to the experimental values. In fact, there are no experimental data reported on formoyl azide.
MO calculations were carried out using the SCFHF ab initio procedure, as well as the MP method. With HF calculations, a total of 15 basis sets were
Table 1 Calculated equilibrium energy parameters for the studied azides using the HFrTZ ) ) and MP4r6-31G methods Property a
Formoyl
Acetyl
cyclic Total energy Ža.u..
open b
y276.63784 y276.92966 c
Dipole moment Ž D .
Thioformoyl
cyclic b
y276.67049 y276.97222 c
open b
y315.69955 y315.55084 c
cyclic b
y315.72997 y315.58285c
open b
y599.30028 y599.31192 c
y599.29549 b y599.29793 c
3.250 b 3.251c
1.719 b 1.598 c
4.096 b 4.2743 c
1.913 b 1.9492 c
4.117 b 4.111c
2.353 b 2.536 c 1.895 1.221 1.059 2.694 0.00 0.891
Total bond order C–X5 C1–N2 N2–N3 N3–N4 N5X C–HŽCH3. Charge density C N N N X HŽCH3.
1.158 1.774 1.047 1.755 0.886 0.933
1.983 1.123 1.120 2.677
1.171 1.805 1.036 1.760
2.005 1.120 1.123 2.679
0.945
0.961
0.951
1.201 1.767 1.048 1.763 1.023 0.889
q0.101 y0.202 y0.003 0.052 y0.157 0.209
q0.260 y0.351 q0.290 q0.071 q0.389 q0.118
0.183 y0.213 y0.002 0.039 y0.151 0.162
0.318 y0.347 0.284 0.060 y0.395 0.080
y0.195 y0.148 0.014 y0.146 0.211 0.263
y0.140 y0.265 0.286 0.099 y0.214 0.234
EHO MO Ža.u.. ELU MO Ža.u..
y0.4767 0.0964
y0.4266 q0.0944
y0.4488 0.1107
y0.4189 0.1118
y0.4311 0.0708
y0.3543 0.0470
a
X s O or S. HFrTZ ) ) . c MP4-SDQr6-31G ) . b
R.H. Abu-Eittah et al.r Chemical Physics Letters 318 (2000) 276–288
280
Table 2 Calculated equilibrium parameters for formoyl, acetyl and thioformoyl azide using HFrTZ ) ) and MP4-SDQr6-31G ) Geometric parameter a
Formyl cylcic
˚. r ŽC–X. ŽA r ŽC–N. r ŽN2–N3. r ŽN3–N4. r ŽN–X. r ŽC–H.
u ŽXCN. Ž8. u ŽHCN. u ŽCNN. u ŽNNN. u ŽNNX. u ŽNXC. a b
1.304 1.332 b 1.265 1.286 b 1.365 1.405 b 1.215 1.364 1.458 b 1.066 1.066 b 112.44 112.25 b 128.05 103.52 104.50 b 111.62
Acetyl open 1.183 1.214 b 1.388 1.392 b 1.263 1.291b 1.077 1.099 b
cyclic
Thioformyl open
1.310 1.330 b 1.271 1.293 b 1.362 1.397 b 1.215
1.187 1.221b 1.402 1.403 b 1.265 1.287 b 1.079 1.099 b
1.366 1.457 b
107.77 104.61 104.51b
111.31 111.61b 128.65
123.56 123.19 b 111.59
104.19 105.11b 111.72
111.53 114.07 b 175.51 172.11b
107.55 105.20 105.12 b
1.705 1.699 b 1.272 1.277 b 1.360 1.358 b 1.226 1.689 1.679 b 1.069 1.071b
1.083 1.074 b 126.23 126.20 b 109.84 110.11b 111.50 114.46 b 175.27 170.09 b
cyclic
112.47 112.31b 122.74 122.81b 111.00 110.965 b 116.84
open 1.631 1.628 b 1.360 1.362 b 1.272 1.275 b 1.076 1.086 b
1.074 1.076 b 129.67 129.69 109.01 108.93 b 113.73 113.75 b 173.65 173.20 b
111.96 88.22 88.68 b
X s O or S. Results of MP4-SDQr6-31G ) .
The calculated bond orders, using the TZ ) ) basis set, reflect the type of bonding in the molecule.
A value of 1.983 between carbon and oxygen indicates an almost pure C5O group, whereas a value of 2.697 between N3 and N4 indicates a bond resonating between double and triple ones and values of
1.123 and 1.120 between the atoms C, N3 and N2, N3 indicates almost pure single bonds. Hence, the possible resonating structures are only I and III, not I and II. The charge distribution on HCON3 ŽTable 1. leads to a value of 1.719 D for the dipole moment of the molecule which is almost twice that of HN3 Ž m s 0.847 " 0.005 D. w23x.
HCON3 is a stronger dipole as a result of the alignment of the two electric dipoles in the molecule, those of C5O and the N3 group.
R.H. Abu-Eittah et al.r Chemical Physics Letters 318 (2000) 276–288
A schematic representation of the optimized geometry of HCON3 is shown: Ža. TZ ) ) and Žb. MP4-SDQ.
These results are informative. A typical free C–H ˚ that calculated in single bond length is 1.14 A, HCON3 is quite reasonable on the basis of sp 2 ˚ hybridization on C-atom. A typical C5O is 1.23 A in length and compares nicely with that calculated ˚ .. The calculated N–N bond for HCON3 Ž1.183 A length in HCON3 compares nicely with that between ˚ for the N–N single free N–N atoms, which is 1.45 A ˚ for the N5N double bond and 1.10 A˚ bond, 1.25 A for the N[N triple bond w24x. A comparison of bond length and bond angle in CH 3 N3 and HCON3 is interesting.
˚ w25x is slightly The C–N bond length of 1.47 A ˚ longer compared to 1.388 A in HCON3 . In CH 3 N3 , sp 3 hybridization is expected on C-atom. 3.1.2. Cyclic conformer(1,2,3,4-oxatriazole) 3.1.2.1. Ground state. Two types of nuclei with an oxygen and three N-atoms are possible, namely
281
1,2,3,4-oxatrizoles Ž1. and 1,2,3,5-oxatriazoles Ž2.. The neutral aromatic species have not yet been reported w26x but 1,2,3,4-oxatriazolium salt Ž3. and mesoionic species Ž4. are known.
The synthesis of oxatriazoles was reported as early as 1896 w27x, but the structure was first proved in 1994 w28x. A large number of publications appeared in 1979 and a limited number appeared in the period 1980–1994. The mesoionic species possesses pesticidal and herbicidal properties and shows varying hypotensive, antiplatelet, fibrinolytic, thrombolytic or bronchiolytic activity. The neutral 1,2,3,4-oxatriazoles Ž1. still await synthesis but some of their properties have been predicted by theoretical calculations w29x.
The oxatriazoles and 1,2,3,5-thiatriazoles, which have not been prepared, are calculated to be in the group with the lowest aromaticity. MO calculations were carried out using different basis sets and methods on the cyclic conformer of formoyl azide, i.e. on 1,2,3,4-oxatriazole. Results are given in Tables 1 and 2. The numbering of atoms, used in MO calculations, is as shown. The results indicate that the open conformer, formoyl azide, is more stable than the cyclic conformer, 1,2,3,4oxatriazole, and D E for conversion: open cyclic is of the order of 21 kcal moly1 ŽTZ ) ) . and ; 30 kcal moly1 when using the MP2 method, but is 27 kcal moly1 when the MP4-SDQ method is used. Also, the cyclic conformer is a stronger dipole Ž m s 3.250 D ŽTZ ) ) . and 3.279 D. using MP4-SDQ than the corresponding open form Ž m s 1.719 D ŽTZ ) ) .
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R.H. Abu-Eittah et al.r Chemical Physics Letters 318 (2000) 276–288
282
˚ Lowering the gradient to O–N bond reaches ; 2 A. a minimum, for large basis sets, is expensive and time consuming because of the low convergence in the second-order SCF calculations Žto obtain minimum energy.. To get a low gradient, ŽEErEr ., in a reasonable time, one has to use a small basis set; this led to the use of the 6-31G basis set rather than any larger one. Table 3 shows how the geometry varies through some of the 16 steps followed to go from the cyclic to the open structure. In Table 3 one shows how the values of energy, bond length and bond angle vary as a function of r ŽN–O. distance through the cyclic open conversion represented in Ž1.. Elongation of the distance r ŽN–O. puts a strain on the molecule and the total energy increases. Maxi˚ which mum strain is obtained at r ŽN–O. s 1.755 A represents the transition state. The geometric parameters of the transition state are shown in Table 3 ˚ the r ŽC–O. where one finds r ŽN–O. s 1.755 A, ˚ Žapproaching the C5O bond and bond is 1.275 A shorter than equilibrium bond length in oxatriazole, ˚ .; the r ŽC–N. bond is 1.324 A˚ approaching a 1.312 A C–N single bond and longer than equilibrium bond ˚ Both N–N bond length in oxatriazole, 1.286 A. lengths are shorter at the transition state compared to the equilibrium bond length in oxatriazole and the terminal N–N bond length is typical for the N[N ˚ the equilibrium geomebond. At r ŽN–O. s 3.435 A,
and 1.810 D. using MP4-SDQ. The charge distribution on the different atoms of the two tautomers is quite different.
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3.1.2.2. Cyclic–open conÕersion. The cyclic–open conversion of formoyl azide has been investigated theoretically. One starts with the cyclic conformer in its best geometry, as obtained with TZ ) ) basis set, and increases the O–N bond stepwise by small intervals. At each increment, the bond length is frozen and geometry optimization of the molecule is carried out.
Freezing the bond length causes the gradient ŽEErEr . to increase as high as ; 50, especially when the
Table 3 Variation of energy and geometry parameters along the reaction coordinate for the conversion of 1,2,3,4-oxatriazole as a function of r ŽN–O.
˚. Geometrical parameters Distance, r ŽN6 –O1 . ŽA 1.457 a Total E Ža.u..
˚. r ŽC2–O1. ŽA r ŽC2–N3. r ŽN3–N4. r ŽN4–N6. u ŽO1C2N3. Ž8. u ŽH5C2N3. u ŽC2N3N4. u ŽN3N4N6. a
1.700
1.760
1900
2.500
3.400
3.435 c
y276.326399 y276.358333 y276.358025 y276.358029 y276.36034 y276.388942 y276.408626 y276.408656 1.332 1.286 1.405 1.228 112.25 127.36 104.49 112.91
1.285 1.315 1.394 1.188 116.76 121.23 103.67 119.59
Equilibrium bond length of cyclic conformer. Transition state. c Equilibrium geometry in formoyl azide. b
1.755 b
1.275 1.324 1.391 1.176 117.67 119.81 103.32 121.51
1.274 1.325 1.390 1.175 117.74 119.69 103.28 121.70
1.250 1.352 1.380 1.148 119.63 116.57 102.24 127.48
1.218 1.404 1.312 1.100 121.59 112.83 102.64 156.41
1.214 1.392 1.292 1.099 125.87 110.31 113.79 171.87
1.214 1.392 1.291 1.099 126.19 110.10 114.46 171.09
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try of the open conformer is obtained whereby results in Table 3 are typical for the open conformer formoylazide. Variation of bond length, bond angle and energy for some points along the reaction coordinate for cyclic open conversion of formoyl azide, given in Table 3, are informative. One clearly sees how bond angles vary: u ŽOCN. from 112.258 to 126.198, u ŽCNN. from 104.498 to 114.468 and u ŽNNN. from 112.98 to 171.098, as one goes from oxatriazole to formoyl azide. It is important to find the dihedral angle u - N6O1C2H5); 1808 at all the conversion steps, indicating that such a process proceeds while the molecule adopts a planar conformer. The best geometry of the open structure is ˚ the best obtained when r ŽN–O. is put at 3.435 A, values of bond length and bond angle. It is important to find that u ŽNNN. f 171.18, i.e. the azide group is not completely linear. The u ŽH5C2N3. is 110.18, indicating a distorted sp 2 hybridization for the Catom. A potential energy ŽHFr6-31G. diagram for the conversion cyclic open structure of formoyl azide is given in Fig. 1. From Fig. 1 and Table 3, one finds that the activation energy for this process is only 2.75 kcal moly1 . Such a result means that 1,2,3,4oxatriazole, mostly, cannot be formed and, if formed, changes immediately to the open tautomer. On the other hand, using different methods, the energy of
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Fig. 1. Variation of the total energy, using the 6-31G basis set, for the cyclic–open conversion of formoyl azide taking the N–O Žbond length. as a reaction coordinate.
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283
conversion open cyclic tautomer is 20.48 kcal moly1 ŽHFr6-31G., ; 30 kcal moly1 ŽMP2., and ; 27 kcal moly1 ŽMP4-SDQ.. In all cases, the results mean that such a process is endothermic. 3.2. Acetyl azide 3.2.1. Open conformer Ab initio MO calculations were performed on the ground state of acetyl azide as was done with formoyl azide.
Of the 15 basis sets used, the results using the TZ ) ) basis set were considered the best Žlowest total energy. and much nearer to the results obtained using MP methods. The results are given in Tables 1 and 2. The calculated dipole moments, 1.913 D ŽTZ ) ) . and 1.949 D ŽMP-SDQ., are much lower than that of the corresponding 1,2,3,4-oxatriazole. Some conclusions can be drawn from the results given in Tables 1 and 2. The C1–C6 bond order is 0.951, indicating a single bond character, whereas that between C and O is 2.005 indicating a pure double bond character, hence resonance structures containing a C–O single bond are inhibited. The bond orders between C and N2 and between N2 and N3 are 1.120 and 1.123, respectively, which indicate a significant single bond character. However, the N3–N4 bond order Ž2.679. indicates a bond between the double and the triple ones. These results mean that N3 should carry a positive charge whereas N2 should carry a negative charge. Calculations give y0.347 and q0.284 as the charges on N2 and N3, respectively. The above results correspond nicely to the values of bond lengths obtained using the TZ ) ) basis sets or MP4-SDQ method ŽTable 2.. It is clear that in acetyl azide the C–O bond length is that of a C5O, sp 2-C, and the N–N bond lengths are equal to those between free N-atoms, i.e. the electron withdrawing effect of the O-atom is retarded by the electron withdrawing ef-
284
R.H. Abu-Eittah et al.r Chemical Physics Letters 318 (2000) 276–288
fect of the N3 group. Consequently the effect of N3 on CO is at a minimum and one gets a bond angle u ŽOCN. of 123.568 and a bond angle u ŽNNN. of 175.518, which means an inhibition of the mutual effect of one group on the other. To account for electron correlation, the MP4-SDQ method of calculation was used; the energy of the ground state as well as the parameters of the equilibrium geometry were calculated; the results are given in Tables 1 and 2. One observes that the molecular electron energy has decreased to y315.582847 a.u. ŽMP4-SDQ. and the electron correlation energy is y0.5672256 a.u. The equilibrium geometry parameters, bond length and angles calculated by the MP4SDQ procedures showed slight differences from those calculated by the HFrTZ ) ) procedures.
p-electron delocalization is predominant and this behavior becomes evident on comparing charge density distribution in the two tautomers: CH 3 CON3 and the corresponding oxatriazole. Table 2 shows that the equilibrium parameters calculated by HFrTZ ) ) and MP4-SDQr6-31G are comparable. 3.2.2.1. Cyclic–open conÕersion. The conversion of 2-methyl oxatriazole to acetyl azide has been investigated theoretically. Starting with the optimized geometry which has the parameters r ŽN–O. s 1.456 ˚ r ŽC–O. s 1.338 A˚ and u ŽOCN. s 111.168, one A, increases the r ŽN–O. distance, freezes and calculates the best conformation at the specific r ŽN–O. distance, then repeats a total of 15 increments; some of the results are given in Table 4.
3.2.2. Cyclic conformer The cyclic conformer of acetyl azide, i.e. methyloxatriazole, has not yet been prepared.
HF–ab initio calculations carried out on the ground state give y315.69955 a.u. for the total energy compared to y315.72997 a.u. for CH 3 CON3 which is more stable than the oxatriazole. Perturbation methods gave y315.550842 a.u. for the total energy when MP4-SDQr6-31G was used. The correlation energy is y0.150482 a.u. The energy of the equilibrium ground state of CH 3 CON3 is y315.582847 a.u. and the correlation energy is y0.567256 a.u. The calculated bond lengths and bond angles are given in Table 2. The calculated dipole moment, 4.096 D ŽHFrTZ ) ) . and 4.274 D ŽMP4-SDQ., is more than twice that calculated for CH 3 CON3 . Calculated values of bond length and bond order indicate significant single bond character for N3–N4, O1–N9 and C–C bonds whereas the double bond character is evident in the C2–N3 and N4–N9 bonds. However, the results indicate that none of the bonds is a pure single or a pure double bond, but intermediate characters predominate. This result means that
As the r ŽN–O. bond length is increased, the total energy increases, N–O bond order decreases and N4–N9 bond order increases. In addition, u ŽOCN. increases. The total energy has a maximum value of ˚ giving an y315.395170 a.u. at r ŽN–O. s 1.780 A, y1 activation energy of 3.25 kcal mol . Hence, the cyclic conformer is easily opened. A potential energy diagram for the cyclic open conversion of acetyl azide is shown in Fig. 2; using different methods, the conversion energies are y19.08 kcal moly1 ŽHFr631G. and y20.08 kcal moly1 ŽMP4-SDQ.. The way in which the equilibrium parameters vary through the change methyl oxatriazole to acetyl azide Žcyclic open. conversion is clearly seen in ˚ in the equilibTable 4. The r ŽC–O. bond is 1.33 A rium cyclic conformer and shortens gradually to 1.22 ˚ in the equilibrium conformer of CH 3 CON3 . On A ˚ Žalmost the other hand, the r ŽC–N. bond is 1.293 A . a double C5N bond in the equilibrium geometry of methyl oxatriazole and elongates gradually to 1.403 ˚ Žalmost a single C–N bond. in the equilibrium A geometry of CH 3 CON3 . Other variations are equally ˚ important; the variation of r ŽN4–N9. from 1.230 A Žsingle bond. in the equilibrium cyclic conformer to
™
™
R.H. Abu-Eittah et al.r Chemical Physics Letters 318 (2000) 276–288
285
Table 4 Variation of energy and geometry parameters along the reaction coordinate for the conversion of 2-methyl 1,2,3,4-oxatriazole as a function of r ŽN–O.
˚. Geometrical parameter Distance, r ŽN9–O1. ŽA 1.457 a Total E Ža.u..
˚. r ŽC2–O1. ŽA r ŽC2–N3. r ŽN3–N4. r ŽN4–N9. u ŽO1C2N3. Ž8. u ŽC2N3N4. u ŽN3N4N9.
1.700
1.780 b
1.785
2.000
2.520
3.359 c
y315.400630 y315.395743 y315.395170 y315.395179 y315.400348 y315.42539 y315.442316 1.338 1.293 1.397 1.230 111.16 105.106 113.10
1.291 1.325 1.380 1.190 115.30 104.52 119.89
1.275 1.340 1.373 1.175 116.45 104.05 122.80
1.275 1.341 1.373 1.174 116.52 104.02 122.99
1.243 1.384 1.349 1.135 118.54 102.61 132.86
1.224 1.416 1.302 1.101 119.52 103.55 158.10
1.220 1.403 1.287 1.099 123.19 114.06 172.10
a
Equilibrium bond length of cyclic conformer. Transition state. c Equilibrium geometry in formoyl azide. b
˚ a clear triple bond in the equilibrium 1.099 A, geometry of CH 3 CON3 . Variation of bond angle along the reaction coordinates for cyclic–open conversion is very illustrative ŽTable 4.. The angle u ŽOCN. varies from 111.168 in methyl oxatrizone to 123.248 in CH 3 CON3 , the angle u ŽCNN. varies from 105.118 in methyl oxatriazole to 114.068 in CH 3 CON3 , and the angle u ŽNNN. varies from 113.108 in methyl oxatriazole to 172.108 in
Fig. 2. Variation of the total energy, using the 6-31G basis set, for the cyclic–open conversion of acetyl azide taking the N–O Žbond length. as a reaction coordinate.
CH 3 CON3 . The N3 group is slightly bent in CH 3 CON3 . 3.3. Thioformoyl azides 3.3.1. Open conformer It is important to compare both the energetic and structural parameters of thioformoyl azide Žnot yet prepared. with those of formoyl azide and the parameters of 1,2,3,4-thiatriazole with those of 1,2,3,4-oxatriazole. Ab initio MO calculations using the HF and MP methods were carried out on the open as well as cyclic conformers of thioformoyl azide using different basis sets. Tables 1 and 2 give the results. The perturbation methods of calculation, MP-SDQr631G ) , gave a correlation energy of y0.092274 a.u. for the cyclic conformer and y0.086216 a.u. for the open conformer. AM1 calculations were performed on 1,2,3,4- and 1,2,3,5-thiatriazole w30x. 5-Thio1,2,3,4-thiatriazole was originally thought to possess an open structure, azidodithiocarbonic acid, but Liebr et al. w31x concluded from infrared spectroscopic measurements that it actually possesses the closed 1,2,3,4-thiatriazole structure. The structure of . has 1,2,3,4-thiatriazole-5-thiolate anion ŽCS 2 Ny 3 w x been examined by Conti et al. 32 using the MNDO method.
R.H. Abu-Eittah et al.r Chemical Physics Letters 318 (2000) 276–288
286
The closed structure Ža. was found to be more stable, D Hf s y5.4 kcal moly1 , than the open structure Žb., D Hf s 31.4 kcal moly1 .
ab initio calculation are given in Že. and Tables 1 and 2.
In this work, MO calculations using the ab initio method were performed on the thioformoyl azide and its cyclic conformers, 1,2,3,4-thiatriazole. The cyclic conformer is slightly more stable, with lower total energy HF or MP methods than the open conformer. The same result, based on the calculated heat of formation, has been obtained before w32x. The crystal structure of 5-amino-1,2,3,4-thiatriazole Žc. has been determined by Zaworotko et al. w33x, comparison of CNDO w34x and SCF–MO–CNDO calculations w35x based on the unknown unsubstituted 1,2,3,4-thiatriazole Žd. is shown, and our results of
Results in Table 2 give the bond length and bond angle for the equilibrium geometry of the cyclic and open conformers of the thioformoyl azide using HF
Table 5 Variation of energy and geometry parameters along the reaction coordinate for the conversion of 1,2,3,4-thiatriazole to thioformoyl azide as a function of r ŽN–S.
˚. Geometrical parameter Distance, r ŽN6–S1. ŽA 1.679 a Total E Ža.u..
˚. r ŽC2–S1. ŽA r ŽC2–N3. r ŽN3–N4. r ŽN4–N6. u ŽS1C2N3. Ž8. u ŽC2N3N4. u ŽN3N4N6. a
1.85
2.300
2.300
2.400 b
2.700
3.570 c
y599.219643 y599.211948 y599.183655 y599.182848 y599.182852 y599.191152 y599.205357 y599.211715 1.699 1.277 1.358 1.234 112.31 110.96 116.15
1.695 1.277 1.365 1.212 115.67 110.96 119.06
1.656 1.319 1.347 1.138 124.74 108.06 132.04
™
1.645 1.338 1.327 1.121 125.80 106.76 137.86
1.644 1.339 1.329 1.121 125.80 106.75 137.93
Equilibrium geometry of 1,2,3,4-thiatriazole Žcyclic conformer of thiaformoyl azide.. Transition state for the conversion: cyclic open structures for thioformoyl azide. c Equilibrium geometry of thioformoyl azide. b
2.401
1.628 1.373 1.283 1.093 126.24 105.12 156.18
1.626 1.370 1.273 1.084 126.64 107.76 173.66
1.627 1.362 1.275 1.085 129.69 113.74 173.20
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287
and MP methods. Correspondence between the values calculated by both methods is quite satisfactory. Our results compare nicely with those obtained by X-ray measurements for the 5-amino-1,2,3,4thiatriazole. 3.3.2. Cyclic–open conÕersion The conversion Ž2. has been investigated theoretically, and the results are given in Table 5.
The process of conversion starts with the optimized geometry of 1,2,3,4-thiatriazole and the N–S bond is elongated stepwise. At each step, the N–S bond is frozen and the optimized geometry is calculated as well as all the physical parameters; some results are given in Table 5. At the optimized geometry of the cyclic con˚ former 1,2,3,4-thiatriazole, the N–S bond is 1.679 A whereas the total energy is y599.219648 a.u. Elon˚ identifies the gation of that bond stepwise to 2.400 A transition state of such a conformer whereby a maximum total energy is obtained: y599.182848 a.u. ˚ Increasing the N–S bond from 2.400 to 2.401 A begins lowering the total energy and, at a distance ˚ the N–S bond is completely r ŽN–S. s 3.100 A, ruptured, giving an N–S bond order of 0.000. Variation of bond angle along the reaction coordinate of cyclic open conversion is given in Table 5. The parameters of the optimized geometry of the open structure of HCSN3 are
™
The azide group is approximately linear and the molecule is planar, the dihedral angle - N6N4N3C2 ); 1808. The activation energy for the conversion 1,2,3,4-thiatriazole thioformoyl azide is 23.09
™
Fig. 3. Variation of the total energy, using the 6-31G basis set, for the cyclic–open conversion of thioformoyl azide taking the N–O Žbond length. as a reaction coordinate.
kcal moly1 ŽFig. 3. which is much larger than for the same conversion of 1,2,3,4-oxatriazole. Hence one concludes that 1,2,3,4-thiatriazole is more stable than thioformoyl azide whereas formoyl azide is more stable than 1,2,3,4-oxatriazole.
4. Conclusions The attachment of an azide group to a carbonyl group in acyl azides plays a unique and significant role in the structure, geometry, stability and reactivity of these compounds. The first members of this series of compounds have not yet been prepared and a theoretical investigation of their structure is needed. The results obtained in this work show that formoyl, thioformoyl and acetyl azides have a pure covalent C5O group with no ionic resonating structures in addition to an approximate linear azide group. The dipole moment vectors of the two groups are in the same direction and this leads to a compound which is a strong dipole. The cyclic conformers of formoyl, acetyl and thioformoyl azides are 1,2,3,4-oxatriazole, 5-CH 31,2,3,4-oxatriazole and 1,2,3,4-thiatriazole. MO calculations using both HF and MP methods have shown that formoyl and acetyl azides are more stable than the corresponding oxatriazoles and the activation energy of cyclic open conversion is of the order
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R.H. Abu-Eittah et al.r Chemical Physics Letters 318 (2000) 276–288
of 3 kcal moly1 whereas the energy of conversion is of the order of 23 kcal moly1 . The situation is different in the case of thioformoyl azide. The results of this work indicate that 1,2,3,4-thiatriazole is more stable than thioformoyl azide. The activation energy of the conversion thiatriazole thioformoyl azide is 23 kcal moly1 whereas the energy for the same conversion is only 5 kcal moly1 . MO calculations using the perturbation methods generally gave a slightly lower total energy than that obtained with the HF method. The equilibrium ground state of formoyl azide has a total energy of y276.670490 a.u. when the HFrTZ ) ) method is used and has a total energy of y276.972217 a.u. when the MP-SDQr6-31G method is used; the correlation energy is y0.563561 a.u. Acetyl azide has a total energy of y315.729970 a.u. ŽHFr6-31G. for the equilibrium ground state compared to y315.582847 a.u. ŽMP4-SDQr6-31G ) . with a correlation energy of y0.140531 a.u. The total energy of thioformoyl azide is y599.295493 a.u. ŽHFr TZ ) ) . compared to y599.297931 a.u. ŽMP4-SDQr 6-31G ) . with a correlation energy of y0.08646 a.u.
™
References w1x B.L. Evans, P. Gray, A.D. Yoffe, Chem. Rev. 59 Ž1959. 515. w2x P. Gray, Q. Rev., Chem. Soc. 17 Ž1963. 441. w3x M. Green, in: C.B. Colburn ŽEd.., Developments in Inorganic Nitrogen Chemistry, vol. I, Elsevier, Amsterdam, 1966, p. 1. w4x A.D. Yoffe, in: C.B. Colburn ŽEd.., Developments in Inorganic Nitrogen Chemistry, vol. 1, Elsevier, Amsterdam, 1966, p. 72. w5x R. Abu-Eittah, A. Elshhawy, J. Appl. Spectrosc. 26 Ž2. Ž1972. 270. w6x K.R. Bhaskar, Indian J. Chem. Eng. 4 Ž8. Ž1966. 368. w7x Ya.K. Syrkin, E.A. Shott-L’vova, Dokl. Akad. Nauk S.S.S.R. 87 Ž1952. 639. w8x H.O. Spauschus, J.M. Scott, J. Am. Chem. Soc. 73 Ž1951. 210.
w9x I.N. Levine, Quantum Chemistry, 4th edn., Prentice-Hall, Englewood Cliffs, NJ, 1991, p. 402. w10x M.W. Schmidt, K.K. Baldridge, J.A. Boatz, S.T. Elbert, M.S. Gordon, J.H. Jensen, S. Koseki, N. Matsunage, K.A. Nguyen, S.J. Su, T.L. Windus, M. Dupuis, J.A. Montgomery, J. Comput. Chem. 14 Ž1993. 1347. w11x R. Krishnm, J.A. Pople, Int. J. Quantum Chem. 14 Ž1978. 91. w12x R.J. Bartlette, Am. Rev. Phys. Chem. 32 Ž1981. 359. w13x J.A. Pople, W.J. Hehre, J. Comput. Phys. 27 Ž1978. 161. w14x R. Ditchfield, W.J. Hehre, J.A. Pople, J. Chem. Phys. 54 Ž1971. 724. w15x W.J. Hehre, R. Ditchfield, J.A. Pople, J. Chem. Phys. 56 Ž1972. 2257. w16x P.C. Hariharan, J.A. Pople, Theor. Chim. Acta 28 Ž1973. 213. w17x R. Krishnan, J.S. Binkley, R. Seeger, J.A. Pople, J. Chem. Phys. 72 Ž1980. 650. w18x T.H. Dunning, Jr., P.J. Hay, in: H.F. Shaefer III ŽEd.., Methods of Electronic Structure Theory, ch. 1, Plenum, New York, 1977, p. 1. w19x T.H. Dunning, J. Chem. Phys. 55 Ž1971. 716. w20x J. Thiele, O. Stange, Ann. Chem. 283 Ž1984. 1. w21x J. Thiele, O. Stange, Chem. Ber. 27 Ž1984. 31. w22x T. Curtius, K. Heidenreich, Chem. Ber. 27 Ž1984. 55. w23x E. Amble, B.P. Dailey, J. Chem. Phys. 18 Ž1950. 1422. w24x M. Green, in: C.B. Colburn ŽEd.., Developments in Inorganic Nitrogen Chemistry, vol. I, Elsevier, Amsterdam, 1966, p. 1. w25x R.L. Livingston, C.N.R. Rao, J. Phys. Chem. 41 Ž1964. 999. w26x M. Begtrup, in: A.R. Katritzky, C.W. Rees, E.F.V. Scriven ŽEds.., Comprehensive Heterocyclic Chemistry, II, vol. 4, Pergamon, Oxford, 1995, p. 679. w27x H.V. Pechmann, Ber. Dtsch. Chem. Ges. 29 Ž1896. 2161. w28x W.V. Farrar, J. Chem. Soc. 906 Ž1964. . w29x A.R. Katritzky, P. Barczynski, J. Prakt. Chem. 332 Ž1990. 885. w30x A.R. Katritzky, P. Barczynski, J. Prakt. Chem. 332 Ž1990. 885. w31x E. Lieber, C.N. Pillai, J. Ramachandran, R.D. Hites, J. Org. Chem. 22 Ž1957. 1750. w32x M. Conti, D.W. Franco, M. Trsic, Inorg. Chim. Acta 113 Ž1986. 71. w33x A. Holm, B. Due Larsen, in: A.R. Katritzky, C.W. Rees, E.F.V. Scriven ŽEds.., Comprehensive Heterocyclic Chemistry, II, vol. 4, Pergamon, Oxford, 1995, p. 694. w34x A. Holm, Adv. Heterocycl. Chem. 20 Ž1976. 145. w35x R.J. Boyd, M.A. Whitehead, J. Chem. Soc., Dalton Trans. 73 Ž1972. .
Chemical Physics Letters 318 Ž2000. 276–288 www.elsevier.nlrlocatercplett
The electronic structure of some acyl azides: cyclic–open tautomerism Rafie H. Abu-Eittah ) , Hussein Moustafa, Ahmad M. Al-Omar Department of Chemistry, Faculty of Science, Cairo UniÕersity, Giza, Egypt Received 20 January 1999; in final form 12 November 1999
Abstract Ab initio molecular orbital calculations including correlation interaction have been performed on formoyl, thioformoyl and acetyl azides using fairly large basis sets. The structural and energetic parameters were calculated at optimized geometry. Cyclic–open tautomerism has been theoretically investigated and the results showed that formoyl azide and acetyl azide are more stable than the corresponding cyclic conformers, oxatriazoles, whereas 1,2,3,4-thiatriazole is more stable than thioformoyl azide. The process of cyclic open conversion of the studied compounds was investigated theoretically and the energy of activation of such process was calculated. q 2000 Elsevier Science B.V. All rights reserved.
™
1. Introduction The reviews by Evans et al. w1x and by Gray w2x give very valuable surveys on the physical properties and thermodynamic and decomposition processes of inorganic azides. Bonding in nitrogen compounds w3x and in inorganic azides w4x was discussed. The electronic structures as well as molecular orbital ŽMO. calculations were performed on some benzene azides w5x. Most of the reported studies are not up to date and accurate theoretical work on the electronic structure and properties of azides is still missing. As for carbonyl azide, the situation is less satisfactory. In fact, the first ultraviolet ŽUV. spectrum of
) Corresponding author. Fax: q20-202-5727556; e-mail: [email protected]
benzoyl azide is reported in 1966 by Bhaskar w6x. Attachment of a aC5O group to an azide group
will affect the geometry, reactivity and structure of the azide group and hence the acyl azide molecule. This is clear when comparing the dipole moment of C 6 H 5 N3 which is 1.44 D w7x, using the benzene as a solvent, with that of C 6 H 5 CON3 which is 2.60 D w7x in the same solvent. Along the same lines, the dipole moment of CH 3 COCH 2 N3 is 3.64 D w8x in benzene as a solvent whereas that of CH 2 5CH–CH 2 –N3 is 1.92 D w7x. The first members of acyl azides, HCON3 , CH 3 CON3 , C 2 H 5 CON3 , or their cyclic conformers, 1,2,3,4-oxatriazole, 5-methyl- or 5-ethyl-1,2,3,4oxatriazole, have not yet been prepared. In this work, a theoretical treatment on a non-empirical basis has
0009-2614r00r$ - see front matter q 2000 Elsevier Science B.V. All rights reserved. PII: S 0 0 0 9 - 2 6 1 4 Ž 9 9 . 0 1 3 3 1 - 7
R.H. Abu-Eittah et al.r Chemical Physics Letters 318 (2000) 276–288
been carried out to investigate the conformations, geometry and electronic structure of these molecules. MO calculations using the ab initio method and different basis sets of which are 6-31G ) , 6-311G, 6-311G ) , DZ, DZ ) , TZ, TZ ) and TZ ) ) were tried. As the basis set is expanded, including d-functions as in the starred basis sets and both d- and p-functions as in the double starred sets, the total energy is lowered. To account for electron correlation, MP methods were used. MO calculations were performed using both HF and MP methods on the optimized geometry of the ground state of both the cyclic and open conformers of the studied molecules. The process of cyclic open conversion was investigated theoretically by MO calculations using HFr6-31G method along the reaction coordinate.
™
277
The HF energy of the molecule is nr2
E HF s 2
Ý
nr2 nr2
Hicore q i
is1
Ý Ý Ž 2 Ji j y K i j . q VNN , is1 js1
Ž 6. core < Hˆicore ' ² f i Ž 1 . < HˆŽ1. fi Ž 1. : i
¦
Za
' f i Ž 1 . y 12 =12 y Ý
r1 a
a
;
fi Ž 1. ,
Ž 7.
and
¦
Ji j s f i Ž 1 . f j Ž 2 .
1 r 12
;
fi Ž 1. f j Ž 2. .
Ž 8.
The molecular orbitals f i are normalized and the closed-subshell orthogonal HF MOs satisfy Fˆ Ž 1 . f i Ž 1 . s ´ i f i Ž 1 . ,
Ž 9.
2. Computational methods
where ´ i is the orbital energy and the Fock operator Fˆ is given by
2.1. Electronic states
core Fˆ Ž 1 . s HˆŽ1. q
nr2
For a closed-shell configuration containing n electrons, the molecular Hartree–Fock ŽHF. wavefunction is written as an antisymmetrized product of spin orbitals. The HF molecular energy E HF is w9x: EHF s ² D < Hˆel q VNN < D : ,
Ž 1.
where D is the normalized Slater-determinant HF wavefunction, D s < f 1 . . . f 1 fn fn <
Ž 2.
and ² D < VNN < D : s VNN ² D < D : s V NN .
Ž 3.
V NN is the potential energy due to nuclear repulsion. The electronic Hamiltonian Hˆel is the sum of oneelectron operator, Fˆi , and two-electron operators, gˆ i j Hˆel s Ý Fˆi q Ý i
j
Ý gˆ i j ,
Ž 4.
j)i
where 1 Za Fˆi s y =i2 y Ý 2 a ri a
Ý
2 Jˆj Ž 1 . y Kˆ j Ž 1 . ,
Ž 10 .
js1
and
gˆ i j s
1 ri j
.
Ž 5.
´ i s f i) Ž 1 . Fˆ Ž 1 . f i Ž 1 . dÕ 1 .
H
Ž 11 .
Substitution and summing over the nr2 occupied orbitals gives nr2
E HF s 2
nr2 nr2
Ý ´i y
Ý Ý Ž2 J
is1
is1 js1
ij
yK i j . q VNN .
Ž 12 .
MO calculations were carried out using the GAMESS program, version 1998, distributed free by the authors of the program w10x. Large numbers of basis sets were attempted, the best being that which leads to a minimum total energy of the ground state. One starts with the minimal STO and continues using the split-valence DZ and TZ basis sets which adds porbitals to the H-atom and d-orbitals to atoms other than the H-atom. Addition of d-orbitals affected the value of the energy significantly whereas addition of p-orbitals has a minor effect. Calculations were carried out on a PC Pentium 300 MHz under Windows 95. Actual molecular applications of MP perturbation theory began only in 1975 with the work of Pople and co-workers w11x and Bartlette and co-workers w12x.
278
R.H. Abu-Eittah et al.r Chemical Physics Letters 318 (2000) 276–288
The MP unperturbed Hamiltonian H o is taken as the sum of the one-electron Fock operator, and the ground-state HF wavefunction Fo is one of the zero-order wavefunctions. The perturbation H X is the difference between the true molecular electronic Hamiltonian H and H o . Formulas for the MP energy correction E Ž2., E Ž3., E Ž4., et cetera, have been derived w11x. In MP4 calculations the terms that involve triply substituted determinants is very time consuming and hence are sometimes neglected. A commonly used approximation is designated by MP4-SDQ where SDQ Žsingle, double and quadruple. excitations are involved. 2.2. Basis sets The success of MO calculation depends on the use of an adequate basis set. In the LCAO method each MO is expressed as a finite sum of Slater-type orbitals ŽSTO. or Gaussian-type functions ŽGTF.. Instead of using individual Gaussian functions as basis functions, contracted GTFs, each of which is a linear combination of a number of primitive Gaussian functions, are used. A considerable increase in computational efficiency can be obtained if the components of Gaussian primitives are shared between different basis functions w13x. At the split-valence level this has been exploited by sharing primitive exponents between s- and p-functions for the valence functions. A series of basis sets has been defined and designated K-LMG where K, L and M are integers. Such a basis set for a first-row element consists of an s-type inner-shell function with K Gaussians, an inner-set of valence s- and p-type functions with L Gaussians, and another outer sp set with M Gaussians. Both valence sets have shared exponents. Some such split-valence sets were developed w14,15x and denoted 4-31G, 5-31G and 6-31G. In each case, the valence of a first-row atom is represented by an inner three-Gaussian function and outer single-Gaussian function. The inner shell Žexpect for H-atom. is represented by a single function which is the sum of four, five or six Gaussian functions. Addition of the polarization function gives additional flexibility to the description of molecular orbitals w16x. These polarization functions are second-order Gaussians for non-H-atoms and first-order Žp-type Gaussians. for hydrogen. Such functions have been added to the
6-31G split-valence basis to give the sets 6-31G ) Žpolarization function only on heavy atoms. and 6-31G ) ) Žpolarization functions on heavy and Hatoms.. A contracted Gaussian basis set Ž6-311G ) ) . has been developed by optimizing exponents and coefficients at the Møller–Plesset ŽMP. second-order level for the ground states of first-row atoms w17x. This has a triple splitŽ311. in the s- and p-valence shells associated with a six-Gaussian inner shell together with a single set of uncontracted polarization functions on each atom. This basis set partly overcomes some of the shortcomings of 6-31G, 6-31G ) and 6-31G ) ) basis set. The double-zeta ŽDZ. basis sets were introduced to improve energy calculation w18x. In the DZ basis set, the number of contracted Gaussian-type orbitals ŽCGTO. is twice the number of occupied atomic orbitals ŽAO. that differ in their exponents Ž z .. The DZ basis set for H-atom is Ž3s.rw2sx and for C-, Oand N-atoms is Ž9s, 5p.rw3s, 2px. The triple-zeta ŽTZ. basis set was introduced to describe each AO as a combination of three STOs which differ in their exponents w19x. For H-atom the TZ basis set is Ž5s.rw3sx and for C-, N- and O-atoms is represented by Ž10s, 6p.rw5s, 3px. For heavy atoms such as sulfur ŽS. the TZ basis set is represented by Ž13s, 10p.rw6s, 5px.
3. Result and discussions 3.1. Formoyl azide 3.1.1. Open conformer Although carbamoyl azide
has been known since 1894 w20–22x and many of its derivatives are also known, yet compounds of the general formula
R.H. Abu-Eittah et al.r Chemical Physics Letters 318 (2000) 276–288
where R s H, alkyl or phenyl are scarcely studied in the literature. Such compounds are, generally, not easy to prepare and in many cases are unstable. Formoyl azide is a gas and has not yet been prepared. None of its physical properties or spectral data are known. The cyclic tautomer, oxatriazole, has not been reported. In this work, one investigates theoretically electronic structure, geometry and energies of such groups of compounds.
279
tried, starting with a single minimal basis set STO-6G and ending with a sophisticated one – TZ ) ) . The best basis set, TZ ) ) , gave rise to y276.67049 a.u. for the total energy of HCON3 whereas the STO-6G basis set gave rise to y275.56434 a.u. To account for electron correlations, perturbation methods are used. Møller–Plesset ŽMP. perturbation method were used and the total E, equilibrium geometry and dipole moment were calculated for formoyl azide. The total energy decreased to y272.97797 and y272.97222 a.u. when MP2 and MP4-SDQ methods were used. The calculated electron correlation energy is y0.56356 a.u. The calculated energetic parameters are given in Table 1 whereas the geometrical parameters are given in Table 2. The geometrical variables calculated using the TZ ) ) basis set are comparable with those calculated using the MP method and both are expected to be near to the experimental values. In fact, there are no experimental data reported on formoyl azide.
MO calculations were carried out using the SCFHF ab initio procedure, as well as the MP method. With HF calculations, a total of 15 basis sets were
Table 1 Calculated equilibrium energy parameters for the studied azides using the HFrTZ ) ) and MP4r6-31G methods Property a
Formoyl
Acetyl
cyclic Total energy Ža.u..
open b
y276.63784 y276.92966 c
Dipole moment Ž D .
Thioformoyl
cyclic b
y276.67049 y276.97222 c
open b
y315.69955 y315.55084 c
cyclic b
y315.72997 y315.58285c
open b
y599.30028 y599.31192 c
y599.29549 b y599.29793 c
3.250 b 3.251c
1.719 b 1.598 c
4.096 b 4.2743 c
1.913 b 1.9492 c
4.117 b 4.111c
2.353 b 2.536 c 1.895 1.221 1.059 2.694 0.00 0.891
Total bond order C–X5 C1–N2 N2–N3 N3–N4 N5X C–HŽCH3. Charge density C N N N X HŽCH3.
1.158 1.774 1.047 1.755 0.886 0.933
1.983 1.123 1.120 2.677
1.171 1.805 1.036 1.760
2.005 1.120 1.123 2.679
0.945
0.961
0.951
1.201 1.767 1.048 1.763 1.023 0.889
q0.101 y0.202 y0.003 0.052 y0.157 0.209
q0.260 y0.351 q0.290 q0.071 q0.389 q0.118
0.183 y0.213 y0.002 0.039 y0.151 0.162
0.318 y0.347 0.284 0.060 y0.395 0.080
y0.195 y0.148 0.014 y0.146 0.211 0.263
y0.140 y0.265 0.286 0.099 y0.214 0.234
EHO MO Ža.u.. ELU MO Ža.u..
y0.4767 0.0964
y0.4266 q0.0944
y0.4488 0.1107
y0.4189 0.1118
y0.4311 0.0708
y0.3543 0.0470
a
X s O or S. HFrTZ ) ) . c MP4-SDQr6-31G ) . b
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280
Table 2 Calculated equilibrium parameters for formoyl, acetyl and thioformoyl azide using HFrTZ ) ) and MP4-SDQr6-31G ) Geometric parameter a
Formyl cylcic
˚. r ŽC–X. ŽA r ŽC–N. r ŽN2–N3. r ŽN3–N4. r ŽN–X. r ŽC–H.
u ŽXCN. Ž8. u ŽHCN. u ŽCNN. u ŽNNN. u ŽNNX. u ŽNXC. a b
1.304 1.332 b 1.265 1.286 b 1.365 1.405 b 1.215 1.364 1.458 b 1.066 1.066 b 112.44 112.25 b 128.05 103.52 104.50 b 111.62
Acetyl open 1.183 1.214 b 1.388 1.392 b 1.263 1.291b 1.077 1.099 b
cyclic
Thioformyl open
1.310 1.330 b 1.271 1.293 b 1.362 1.397 b 1.215
1.187 1.221b 1.402 1.403 b 1.265 1.287 b 1.079 1.099 b
1.366 1.457 b
107.77 104.61 104.51b
111.31 111.61b 128.65
123.56 123.19 b 111.59
104.19 105.11b 111.72
111.53 114.07 b 175.51 172.11b
107.55 105.20 105.12 b
1.705 1.699 b 1.272 1.277 b 1.360 1.358 b 1.226 1.689 1.679 b 1.069 1.071b
1.083 1.074 b 126.23 126.20 b 109.84 110.11b 111.50 114.46 b 175.27 170.09 b
cyclic
112.47 112.31b 122.74 122.81b 111.00 110.965 b 116.84
open 1.631 1.628 b 1.360 1.362 b 1.272 1.275 b 1.076 1.086 b
1.074 1.076 b 129.67 129.69 109.01 108.93 b 113.73 113.75 b 173.65 173.20 b
111.96 88.22 88.68 b
X s O or S. Results of MP4-SDQr6-31G ) .
The calculated bond orders, using the TZ ) ) basis set, reflect the type of bonding in the molecule.
A value of 1.983 between carbon and oxygen indicates an almost pure C5O group, whereas a value of 2.697 between N3 and N4 indicates a bond resonating between double and triple ones and values of
1.123 and 1.120 between the atoms C, N3 and N2, N3 indicates almost pure single bonds. Hence, the possible resonating structures are only I and III, not I and II. The charge distribution on HCON3 ŽTable 1. leads to a value of 1.719 D for the dipole moment of the molecule which is almost twice that of HN3 Ž m s 0.847 " 0.005 D. w23x.
HCON3 is a stronger dipole as a result of the alignment of the two electric dipoles in the molecule, those of C5O and the N3 group.
R.H. Abu-Eittah et al.r Chemical Physics Letters 318 (2000) 276–288
A schematic representation of the optimized geometry of HCON3 is shown: Ža. TZ ) ) and Žb. MP4-SDQ.
These results are informative. A typical free C–H ˚ that calculated in single bond length is 1.14 A, HCON3 is quite reasonable on the basis of sp 2 ˚ hybridization on C-atom. A typical C5O is 1.23 A in length and compares nicely with that calculated ˚ .. The calculated N–N bond for HCON3 Ž1.183 A length in HCON3 compares nicely with that between ˚ for the N–N single free N–N atoms, which is 1.45 A ˚ for the N5N double bond and 1.10 A˚ bond, 1.25 A for the N[N triple bond w24x. A comparison of bond length and bond angle in CH 3 N3 and HCON3 is interesting.
˚ w25x is slightly The C–N bond length of 1.47 A ˚ longer compared to 1.388 A in HCON3 . In CH 3 N3 , sp 3 hybridization is expected on C-atom. 3.1.2. Cyclic conformer(1,2,3,4-oxatriazole) 3.1.2.1. Ground state. Two types of nuclei with an oxygen and three N-atoms are possible, namely
281
1,2,3,4-oxatrizoles Ž1. and 1,2,3,5-oxatriazoles Ž2.. The neutral aromatic species have not yet been reported w26x but 1,2,3,4-oxatriazolium salt Ž3. and mesoionic species Ž4. are known.
The synthesis of oxatriazoles was reported as early as 1896 w27x, but the structure was first proved in 1994 w28x. A large number of publications appeared in 1979 and a limited number appeared in the period 1980–1994. The mesoionic species possesses pesticidal and herbicidal properties and shows varying hypotensive, antiplatelet, fibrinolytic, thrombolytic or bronchiolytic activity. The neutral 1,2,3,4-oxatriazoles Ž1. still await synthesis but some of their properties have been predicted by theoretical calculations w29x.
The oxatriazoles and 1,2,3,5-thiatriazoles, which have not been prepared, are calculated to be in the group with the lowest aromaticity. MO calculations were carried out using different basis sets and methods on the cyclic conformer of formoyl azide, i.e. on 1,2,3,4-oxatriazole. Results are given in Tables 1 and 2. The numbering of atoms, used in MO calculations, is as shown. The results indicate that the open conformer, formoyl azide, is more stable than the cyclic conformer, 1,2,3,4oxatriazole, and D E for conversion: open cyclic is of the order of 21 kcal moly1 ŽTZ ) ) . and ; 30 kcal moly1 when using the MP2 method, but is 27 kcal moly1 when the MP4-SDQ method is used. Also, the cyclic conformer is a stronger dipole Ž m s 3.250 D ŽTZ ) ) . and 3.279 D. using MP4-SDQ than the corresponding open form Ž m s 1.719 D ŽTZ ) ) .
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282
˚ Lowering the gradient to O–N bond reaches ; 2 A. a minimum, for large basis sets, is expensive and time consuming because of the low convergence in the second-order SCF calculations Žto obtain minimum energy.. To get a low gradient, ŽEErEr ., in a reasonable time, one has to use a small basis set; this led to the use of the 6-31G basis set rather than any larger one. Table 3 shows how the geometry varies through some of the 16 steps followed to go from the cyclic to the open structure. In Table 3 one shows how the values of energy, bond length and bond angle vary as a function of r ŽN–O. distance through the cyclic open conversion represented in Ž1.. Elongation of the distance r ŽN–O. puts a strain on the molecule and the total energy increases. Maxi˚ which mum strain is obtained at r ŽN–O. s 1.755 A represents the transition state. The geometric parameters of the transition state are shown in Table 3 ˚ the r ŽC–O. where one finds r ŽN–O. s 1.755 A, ˚ Žapproaching the C5O bond and bond is 1.275 A shorter than equilibrium bond length in oxatriazole, ˚ .; the r ŽC–N. bond is 1.324 A˚ approaching a 1.312 A C–N single bond and longer than equilibrium bond ˚ Both N–N bond length in oxatriazole, 1.286 A. lengths are shorter at the transition state compared to the equilibrium bond length in oxatriazole and the terminal N–N bond length is typical for the N[N ˚ the equilibrium geomebond. At r ŽN–O. s 3.435 A,
and 1.810 D. using MP4-SDQ. The charge distribution on the different atoms of the two tautomers is quite different.
™
3.1.2.2. Cyclic–open conÕersion. The cyclic–open conversion of formoyl azide has been investigated theoretically. One starts with the cyclic conformer in its best geometry, as obtained with TZ ) ) basis set, and increases the O–N bond stepwise by small intervals. At each increment, the bond length is frozen and geometry optimization of the molecule is carried out.
Freezing the bond length causes the gradient ŽEErEr . to increase as high as ; 50, especially when the
Table 3 Variation of energy and geometry parameters along the reaction coordinate for the conversion of 1,2,3,4-oxatriazole as a function of r ŽN–O.
˚. Geometrical parameters Distance, r ŽN6 –O1 . ŽA 1.457 a Total E Ža.u..
˚. r ŽC2–O1. ŽA r ŽC2–N3. r ŽN3–N4. r ŽN4–N6. u ŽO1C2N3. Ž8. u ŽH5C2N3. u ŽC2N3N4. u ŽN3N4N6. a
1.700
1.760
1900
2.500
3.400
3.435 c
y276.326399 y276.358333 y276.358025 y276.358029 y276.36034 y276.388942 y276.408626 y276.408656 1.332 1.286 1.405 1.228 112.25 127.36 104.49 112.91
1.285 1.315 1.394 1.188 116.76 121.23 103.67 119.59
Equilibrium bond length of cyclic conformer. Transition state. c Equilibrium geometry in formoyl azide. b
1.755 b
1.275 1.324 1.391 1.176 117.67 119.81 103.32 121.51
1.274 1.325 1.390 1.175 117.74 119.69 103.28 121.70
1.250 1.352 1.380 1.148 119.63 116.57 102.24 127.48
1.218 1.404 1.312 1.100 121.59 112.83 102.64 156.41
1.214 1.392 1.292 1.099 125.87 110.31 113.79 171.87
1.214 1.392 1.291 1.099 126.19 110.10 114.46 171.09
R.H. Abu-Eittah et al.r Chemical Physics Letters 318 (2000) 276–288
try of the open conformer is obtained whereby results in Table 3 are typical for the open conformer formoylazide. Variation of bond length, bond angle and energy for some points along the reaction coordinate for cyclic open conversion of formoyl azide, given in Table 3, are informative. One clearly sees how bond angles vary: u ŽOCN. from 112.258 to 126.198, u ŽCNN. from 104.498 to 114.468 and u ŽNNN. from 112.98 to 171.098, as one goes from oxatriazole to formoyl azide. It is important to find the dihedral angle u - N6O1C2H5); 1808 at all the conversion steps, indicating that such a process proceeds while the molecule adopts a planar conformer. The best geometry of the open structure is ˚ the best obtained when r ŽN–O. is put at 3.435 A, values of bond length and bond angle. It is important to find that u ŽNNN. f 171.18, i.e. the azide group is not completely linear. The u ŽH5C2N3. is 110.18, indicating a distorted sp 2 hybridization for the Catom. A potential energy ŽHFr6-31G. diagram for the conversion cyclic open structure of formoyl azide is given in Fig. 1. From Fig. 1 and Table 3, one finds that the activation energy for this process is only 2.75 kcal moly1 . Such a result means that 1,2,3,4oxatriazole, mostly, cannot be formed and, if formed, changes immediately to the open tautomer. On the other hand, using different methods, the energy of
™
™
Fig. 1. Variation of the total energy, using the 6-31G basis set, for the cyclic–open conversion of formoyl azide taking the N–O Žbond length. as a reaction coordinate.
™
283
conversion open cyclic tautomer is 20.48 kcal moly1 ŽHFr6-31G., ; 30 kcal moly1 ŽMP2., and ; 27 kcal moly1 ŽMP4-SDQ.. In all cases, the results mean that such a process is endothermic. 3.2. Acetyl azide 3.2.1. Open conformer Ab initio MO calculations were performed on the ground state of acetyl azide as was done with formoyl azide.
Of the 15 basis sets used, the results using the TZ ) ) basis set were considered the best Žlowest total energy. and much nearer to the results obtained using MP methods. The results are given in Tables 1 and 2. The calculated dipole moments, 1.913 D ŽTZ ) ) . and 1.949 D ŽMP-SDQ., are much lower than that of the corresponding 1,2,3,4-oxatriazole. Some conclusions can be drawn from the results given in Tables 1 and 2. The C1–C6 bond order is 0.951, indicating a single bond character, whereas that between C and O is 2.005 indicating a pure double bond character, hence resonance structures containing a C–O single bond are inhibited. The bond orders between C and N2 and between N2 and N3 are 1.120 and 1.123, respectively, which indicate a significant single bond character. However, the N3–N4 bond order Ž2.679. indicates a bond between the double and the triple ones. These results mean that N3 should carry a positive charge whereas N2 should carry a negative charge. Calculations give y0.347 and q0.284 as the charges on N2 and N3, respectively. The above results correspond nicely to the values of bond lengths obtained using the TZ ) ) basis sets or MP4-SDQ method ŽTable 2.. It is clear that in acetyl azide the C–O bond length is that of a C5O, sp 2-C, and the N–N bond lengths are equal to those between free N-atoms, i.e. the electron withdrawing effect of the O-atom is retarded by the electron withdrawing ef-
284
R.H. Abu-Eittah et al.r Chemical Physics Letters 318 (2000) 276–288
fect of the N3 group. Consequently the effect of N3 on CO is at a minimum and one gets a bond angle u ŽOCN. of 123.568 and a bond angle u ŽNNN. of 175.518, which means an inhibition of the mutual effect of one group on the other. To account for electron correlation, the MP4-SDQ method of calculation was used; the energy of the ground state as well as the parameters of the equilibrium geometry were calculated; the results are given in Tables 1 and 2. One observes that the molecular electron energy has decreased to y315.582847 a.u. ŽMP4-SDQ. and the electron correlation energy is y0.5672256 a.u. The equilibrium geometry parameters, bond length and angles calculated by the MP4SDQ procedures showed slight differences from those calculated by the HFrTZ ) ) procedures.
p-electron delocalization is predominant and this behavior becomes evident on comparing charge density distribution in the two tautomers: CH 3 CON3 and the corresponding oxatriazole. Table 2 shows that the equilibrium parameters calculated by HFrTZ ) ) and MP4-SDQr6-31G are comparable. 3.2.2.1. Cyclic–open conÕersion. The conversion of 2-methyl oxatriazole to acetyl azide has been investigated theoretically. Starting with the optimized geometry which has the parameters r ŽN–O. s 1.456 ˚ r ŽC–O. s 1.338 A˚ and u ŽOCN. s 111.168, one A, increases the r ŽN–O. distance, freezes and calculates the best conformation at the specific r ŽN–O. distance, then repeats a total of 15 increments; some of the results are given in Table 4.
3.2.2. Cyclic conformer The cyclic conformer of acetyl azide, i.e. methyloxatriazole, has not yet been prepared.
HF–ab initio calculations carried out on the ground state give y315.69955 a.u. for the total energy compared to y315.72997 a.u. for CH 3 CON3 which is more stable than the oxatriazole. Perturbation methods gave y315.550842 a.u. for the total energy when MP4-SDQr6-31G was used. The correlation energy is y0.150482 a.u. The energy of the equilibrium ground state of CH 3 CON3 is y315.582847 a.u. and the correlation energy is y0.567256 a.u. The calculated bond lengths and bond angles are given in Table 2. The calculated dipole moment, 4.096 D ŽHFrTZ ) ) . and 4.274 D ŽMP4-SDQ., is more than twice that calculated for CH 3 CON3 . Calculated values of bond length and bond order indicate significant single bond character for N3–N4, O1–N9 and C–C bonds whereas the double bond character is evident in the C2–N3 and N4–N9 bonds. However, the results indicate that none of the bonds is a pure single or a pure double bond, but intermediate characters predominate. This result means that
As the r ŽN–O. bond length is increased, the total energy increases, N–O bond order decreases and N4–N9 bond order increases. In addition, u ŽOCN. increases. The total energy has a maximum value of ˚ giving an y315.395170 a.u. at r ŽN–O. s 1.780 A, y1 activation energy of 3.25 kcal mol . Hence, the cyclic conformer is easily opened. A potential energy diagram for the cyclic open conversion of acetyl azide is shown in Fig. 2; using different methods, the conversion energies are y19.08 kcal moly1 ŽHFr631G. and y20.08 kcal moly1 ŽMP4-SDQ.. The way in which the equilibrium parameters vary through the change methyl oxatriazole to acetyl azide Žcyclic open. conversion is clearly seen in ˚ in the equilibTable 4. The r ŽC–O. bond is 1.33 A rium cyclic conformer and shortens gradually to 1.22 ˚ in the equilibrium conformer of CH 3 CON3 . On A ˚ Žalmost the other hand, the r ŽC–N. bond is 1.293 A . a double C5N bond in the equilibrium geometry of methyl oxatriazole and elongates gradually to 1.403 ˚ Žalmost a single C–N bond. in the equilibrium A geometry of CH 3 CON3 . Other variations are equally ˚ important; the variation of r ŽN4–N9. from 1.230 A Žsingle bond. in the equilibrium cyclic conformer to
™
™
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285
Table 4 Variation of energy and geometry parameters along the reaction coordinate for the conversion of 2-methyl 1,2,3,4-oxatriazole as a function of r ŽN–O.
˚. Geometrical parameter Distance, r ŽN9–O1. ŽA 1.457 a Total E Ža.u..
˚. r ŽC2–O1. ŽA r ŽC2–N3. r ŽN3–N4. r ŽN4–N9. u ŽO1C2N3. Ž8. u ŽC2N3N4. u ŽN3N4N9.
1.700
1.780 b
1.785
2.000
2.520
3.359 c
y315.400630 y315.395743 y315.395170 y315.395179 y315.400348 y315.42539 y315.442316 1.338 1.293 1.397 1.230 111.16 105.106 113.10
1.291 1.325 1.380 1.190 115.30 104.52 119.89
1.275 1.340 1.373 1.175 116.45 104.05 122.80
1.275 1.341 1.373 1.174 116.52 104.02 122.99
1.243 1.384 1.349 1.135 118.54 102.61 132.86
1.224 1.416 1.302 1.101 119.52 103.55 158.10
1.220 1.403 1.287 1.099 123.19 114.06 172.10
a
Equilibrium bond length of cyclic conformer. Transition state. c Equilibrium geometry in formoyl azide. b
˚ a clear triple bond in the equilibrium 1.099 A, geometry of CH 3 CON3 . Variation of bond angle along the reaction coordinates for cyclic–open conversion is very illustrative ŽTable 4.. The angle u ŽOCN. varies from 111.168 in methyl oxatrizone to 123.248 in CH 3 CON3 , the angle u ŽCNN. varies from 105.118 in methyl oxatriazole to 114.068 in CH 3 CON3 , and the angle u ŽNNN. varies from 113.108 in methyl oxatriazole to 172.108 in
Fig. 2. Variation of the total energy, using the 6-31G basis set, for the cyclic–open conversion of acetyl azide taking the N–O Žbond length. as a reaction coordinate.
CH 3 CON3 . The N3 group is slightly bent in CH 3 CON3 . 3.3. Thioformoyl azides 3.3.1. Open conformer It is important to compare both the energetic and structural parameters of thioformoyl azide Žnot yet prepared. with those of formoyl azide and the parameters of 1,2,3,4-thiatriazole with those of 1,2,3,4-oxatriazole. Ab initio MO calculations using the HF and MP methods were carried out on the open as well as cyclic conformers of thioformoyl azide using different basis sets. Tables 1 and 2 give the results. The perturbation methods of calculation, MP-SDQr631G ) , gave a correlation energy of y0.092274 a.u. for the cyclic conformer and y0.086216 a.u. for the open conformer. AM1 calculations were performed on 1,2,3,4- and 1,2,3,5-thiatriazole w30x. 5-Thio1,2,3,4-thiatriazole was originally thought to possess an open structure, azidodithiocarbonic acid, but Liebr et al. w31x concluded from infrared spectroscopic measurements that it actually possesses the closed 1,2,3,4-thiatriazole structure. The structure of . has 1,2,3,4-thiatriazole-5-thiolate anion ŽCS 2 Ny 3 w x been examined by Conti et al. 32 using the MNDO method.
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286
The closed structure Ža. was found to be more stable, D Hf s y5.4 kcal moly1 , than the open structure Žb., D Hf s 31.4 kcal moly1 .
ab initio calculation are given in Že. and Tables 1 and 2.
In this work, MO calculations using the ab initio method were performed on the thioformoyl azide and its cyclic conformers, 1,2,3,4-thiatriazole. The cyclic conformer is slightly more stable, with lower total energy HF or MP methods than the open conformer. The same result, based on the calculated heat of formation, has been obtained before w32x. The crystal structure of 5-amino-1,2,3,4-thiatriazole Žc. has been determined by Zaworotko et al. w33x, comparison of CNDO w34x and SCF–MO–CNDO calculations w35x based on the unknown unsubstituted 1,2,3,4-thiatriazole Žd. is shown, and our results of
Results in Table 2 give the bond length and bond angle for the equilibrium geometry of the cyclic and open conformers of the thioformoyl azide using HF
Table 5 Variation of energy and geometry parameters along the reaction coordinate for the conversion of 1,2,3,4-thiatriazole to thioformoyl azide as a function of r ŽN–S.
˚. Geometrical parameter Distance, r ŽN6–S1. ŽA 1.679 a Total E Ža.u..
˚. r ŽC2–S1. ŽA r ŽC2–N3. r ŽN3–N4. r ŽN4–N6. u ŽS1C2N3. Ž8. u ŽC2N3N4. u ŽN3N4N6. a
1.85
2.300
2.300
2.400 b
2.700
3.570 c
y599.219643 y599.211948 y599.183655 y599.182848 y599.182852 y599.191152 y599.205357 y599.211715 1.699 1.277 1.358 1.234 112.31 110.96 116.15
1.695 1.277 1.365 1.212 115.67 110.96 119.06
1.656 1.319 1.347 1.138 124.74 108.06 132.04
™
1.645 1.338 1.327 1.121 125.80 106.76 137.86
1.644 1.339 1.329 1.121 125.80 106.75 137.93
Equilibrium geometry of 1,2,3,4-thiatriazole Žcyclic conformer of thiaformoyl azide.. Transition state for the conversion: cyclic open structures for thioformoyl azide. c Equilibrium geometry of thioformoyl azide. b
2.401
1.628 1.373 1.283 1.093 126.24 105.12 156.18
1.626 1.370 1.273 1.084 126.64 107.76 173.66
1.627 1.362 1.275 1.085 129.69 113.74 173.20
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287
and MP methods. Correspondence between the values calculated by both methods is quite satisfactory. Our results compare nicely with those obtained by X-ray measurements for the 5-amino-1,2,3,4thiatriazole. 3.3.2. Cyclic–open conÕersion The conversion Ž2. has been investigated theoretically, and the results are given in Table 5.
The process of conversion starts with the optimized geometry of 1,2,3,4-thiatriazole and the N–S bond is elongated stepwise. At each step, the N–S bond is frozen and the optimized geometry is calculated as well as all the physical parameters; some results are given in Table 5. At the optimized geometry of the cyclic con˚ former 1,2,3,4-thiatriazole, the N–S bond is 1.679 A whereas the total energy is y599.219648 a.u. Elon˚ identifies the gation of that bond stepwise to 2.400 A transition state of such a conformer whereby a maximum total energy is obtained: y599.182848 a.u. ˚ Increasing the N–S bond from 2.400 to 2.401 A begins lowering the total energy and, at a distance ˚ the N–S bond is completely r ŽN–S. s 3.100 A, ruptured, giving an N–S bond order of 0.000. Variation of bond angle along the reaction coordinate of cyclic open conversion is given in Table 5. The parameters of the optimized geometry of the open structure of HCSN3 are
™
The azide group is approximately linear and the molecule is planar, the dihedral angle - N6N4N3C2 ); 1808. The activation energy for the conversion 1,2,3,4-thiatriazole thioformoyl azide is 23.09
™
Fig. 3. Variation of the total energy, using the 6-31G basis set, for the cyclic–open conversion of thioformoyl azide taking the N–O Žbond length. as a reaction coordinate.
kcal moly1 ŽFig. 3. which is much larger than for the same conversion of 1,2,3,4-oxatriazole. Hence one concludes that 1,2,3,4-thiatriazole is more stable than thioformoyl azide whereas formoyl azide is more stable than 1,2,3,4-oxatriazole.
4. Conclusions The attachment of an azide group to a carbonyl group in acyl azides plays a unique and significant role in the structure, geometry, stability and reactivity of these compounds. The first members of this series of compounds have not yet been prepared and a theoretical investigation of their structure is needed. The results obtained in this work show that formoyl, thioformoyl and acetyl azides have a pure covalent C5O group with no ionic resonating structures in addition to an approximate linear azide group. The dipole moment vectors of the two groups are in the same direction and this leads to a compound which is a strong dipole. The cyclic conformers of formoyl, acetyl and thioformoyl azides are 1,2,3,4-oxatriazole, 5-CH 31,2,3,4-oxatriazole and 1,2,3,4-thiatriazole. MO calculations using both HF and MP methods have shown that formoyl and acetyl azides are more stable than the corresponding oxatriazoles and the activation energy of cyclic open conversion is of the order
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R.H. Abu-Eittah et al.r Chemical Physics Letters 318 (2000) 276–288
of 3 kcal moly1 whereas the energy of conversion is of the order of 23 kcal moly1 . The situation is different in the case of thioformoyl azide. The results of this work indicate that 1,2,3,4-thiatriazole is more stable than thioformoyl azide. The activation energy of the conversion thiatriazole thioformoyl azide is 23 kcal moly1 whereas the energy for the same conversion is only 5 kcal moly1 . MO calculations using the perturbation methods generally gave a slightly lower total energy than that obtained with the HF method. The equilibrium ground state of formoyl azide has a total energy of y276.670490 a.u. when the HFrTZ ) ) method is used and has a total energy of y276.972217 a.u. when the MP-SDQr6-31G method is used; the correlation energy is y0.563561 a.u. Acetyl azide has a total energy of y315.729970 a.u. ŽHFr6-31G. for the equilibrium ground state compared to y315.582847 a.u. ŽMP4-SDQr6-31G ) . with a correlation energy of y0.140531 a.u. The total energy of thioformoyl azide is y599.295493 a.u. ŽHFr TZ ) ) . compared to y599.297931 a.u. ŽMP4-SDQr 6-31G ) . with a correlation energy of y0.08646 a.u.
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