Conformational behaviour of phenylpyrimidines. a quantum mechanical study

0040-4OZO/85S3.00+.00

Te~ralwdiaVol. 4l.No.10,~.191Sto1918.1985 Printed inGrr&Briti.

Pergamon RcscLtd.

CONFORMATIONAL BEHAVIOUR OF PHENYLPYRIMIDINES. A QUANTUM MECHANICAL STUDY

VINCENZO BARONE*a. LUIGI COMMISSOb, FRANCESCO LELJ' and NINO RUSSO

aDepartment of Chemistry, University of Naples,via Mezzocannone 4, I-80134 Naples ITALY. b Department of Chemistry. University of Calabria, I-87034 Arcavacata di Rende (CS) ITALY. (Received in UK 29 January 198.5)

Abstract - The conformational behaviour of isomeric phenylpyrimidines has been studied by STO-3G ab-initio computations. The results show that the torsional angle between the two rings increases with the number of H-H vi cinal interactions and a planar equilibrium conformatibn is obtained only in the absence of these interactions (2-phenylpyrimidine). A Fourier expansion of the torsional potential suggests that the first two terms are sufficient for a good fitting, except in the case of two H-H interactions (5-phenylpyrimidine), where the second term dominates and the first and third terms are of the same order of magnitude. l-5 The conformational behaviour of azabiphenyls has been the subject of many experimental and se6-10 Investigations in view of the considerable interest of these ormiempirical quantum-mechanical ganic bases in several fields of fundamental and applied chemistry. Both approaches are not free from difficulties due to the number of parameters to be determined for describing large amplitude internal motions from one side and to the delicate balance between conjugative and steric interac11-13 In principle non-empirical quantum-mechanical computations should tions from the other side. not suffer from these difficulties and the only obstacle to their systematic use is the enormous amount of computational time that would be required in the case of such large molecules if extended basis sets and possibly configuration interaction would be needed. It has, however, been shown that minimal basis set Hartree-Fock computations provide a very good compromise between the relia13-20 bility of the results and the necessary amount of computational time. Our aim is, therefore,to perform ab-initio computations for the most significant azabiphenyls and to analyze, on these grounds, the interplay of conjugative, steric, and electrostatic interactions in determining the 13,20 conformational behaviour of these molecules. Previous studies have shown that azabiphenyls with the same local environment of inter-ring carbon atoms (i.e. the same ortho groups) have very similar equilibrium conformations, irrespective of their composition and that replacement of ortho CH groups by nitrogen atoms alleviates some of the steric hindrance that arises as the two rings become coplanar. Here we shall analyze in a more quantitative way this aspect with special reference to the relative strength of CH...CH and N... CH interactions between ortho groups of the two rings. Phenylpyrimidines are a natural choice in this connection since the three possible isomers are characterized by two N...CH (2-phenylpyrimidine). one N...CH and one CH...CH (I-phenylpyrimidine) and two CH ...CH (5-phenylpyrimidine) interactions. Furthermore, a comparison with the pre13 viously studied class of phenylpyridines would allow to analyze possible modifications induced by the presence of one or two nitrogen atoms in one ring, while the second ring remains imperturbed. In view of previous experience the torsional potential was computed in steps of 30° by the 21 of the GAUSSIAN/SO package and using STO-3C optimized geometries for the py-

STO-3G basis set

1915

V.

1916

BARONE

et al,

013,14 rimidine13 and benzene22 rings with a C-C inter-ring distance of 1.50 A. Further points were computed in some cases for a better definition of equilibrium conformations. The structures and atom numbering of the different isomers are shown in the Figure.

5-phenylpyrimidine

4-phenylpyrimidine

2-phenylpyrimidine

The data reported in Table I show that 2-phenylpyrimidine has a planar equilibrium conformation, whereas the other two isomers are characterized by non-planar equilibrium conformations; the conformational energies at 30° and 60° further suggest that the equilibrium torsional angle of 4-phenylpyrimidine is smaller than that of 5-phenylpyrimidine. Finally the torsional behaviour is essentially governed by the local environment of inter-ring carbon atoms (Cir) since the pairs &phenylpyrimidine/2_phenylpyridine

and 5_phenylpyrimidine/4_phenylpyridine

(which are identical

from this point of view) have very similar conformational energies. Table I. Conformational energies of phenylpyrimidines and corresponding phenylpyridines (AE in kJ/mol) with respect to planar structures. The STO-3C total energies (in a.u.1 of the planar forms are -486.14360, -486.14173 and -486.13443 for the 2-,4-,and !Z-phenylpyrimidine.respectively. MOLECULE

AE(30°)

AE(60=')

AE(90°)

2.5

14.7

22.5

4-phenylpyrimidine 2-phenylpyridine

-4.9 -4.5

2.2 2.8

9.2 a.7

5-phenylpyrimidine 4-phenylpyridine

-14.7 -14.0

-14.2 -13.7

-8.2 -8.1

2- phenylpyrimidine

The conformational energies de(e) of all the isomers with respect to planar forms can be described by the leading terms of the Fourier expansion(l) E(B) = X J"Vj[l-cos(je)]

(1)

and symmetry requirements forbid odd values of j. the different terms of the Fourier expansion are given by: V2 = 2/3(AE(90°)-AE(30°)+AE(600))

; V4 = 4/3 AE(60°)-3/4V2

; v6 = AE(90°)-V2

(2)

where the values of V2 and V4 are independent from the possible inclusion of V6. When an energy minimum

exists together with the extrema at e = OD and 90° its torsional angle is given by emin = l/2 arcos (-v2/4v41

(3)

including only the first two terms and by 01 = l/.%rCOSi[ -vq -(i$ + gvi %2vg)-‘] /6V6) (4) min when also the V6 term is included. several experimental studies on similar molecules have retained l-3,5 and the data of Table II show that this trun only the terms VP and V4 in the above expansion, cation is justified in the case of 2- and 4-phenylpyrimidine. However in the

case

of 5-phenylpyri-

midine the inclusion of the V6 term is mandatory for an accurate fitting of AE(B) and

a

prO@r

lo-

cation of the equilibrium torsional angle since V6 and V2 are of the same order of magnitude. For this molecule the energy barrier at 90" is lower than the energy barrier at 0' and use of V2 and V4 terms only lead to emin <45O, as actually suggested in ref.2 for the analogous case of biphenyl; however, computation of further points at the STO-3G level indicates that

0 min is actually lower

than 45=', as obtained including the V6 term. A deeper analysis of the results obtained can be performed by remembering that the conformational behaviour of azabiphenyls is governed by two factors, namely the r interaction between the rings, which tends to keep the molecules planar (the conjugative factor) and the interactions between non-bonded atoms (of steric and electrostatic origin), which may be generally alleviated by 23 deviation from planarity. The conjugative contribution EC(B) to the torsional potential can be

Conformationalbehaviourofphcnylpyrimidincs

1917

AEc(e) - E=(B) - Ec(O") = K(cos'B - 1) = -K/2(1-~0~28)

(5)

Table II. Calculated potential constants (in kJ/mol) and torsional angles for potential minima. MOLECULE

v2

v4

2-phenylpyrimidine

23.0

4-phanylpyrimidine 2-phenylpyridine

10.9 10.2

!%phenylpyrimidine 4-phenylpyridine

-5.2 -5.2

e

8' min

min

'6

-3.5

0.0

-0.6

0.0

-7.9 -7.4

34.9 34.9

-1.7 -1.5

33.1 33.2

-13.8 -13.1

47.7 47.8

-3.0 -2.9

43.1 43.2

23 where K is proportional to the square of the density matrix element P1 for the inter-ring (Ibond. As a consequence conjugative interactions only contribute to the V2 term in the Fourier expansion, whereas steric and electrostatic interactions (which are functions of the inverse of the interatomic distances between non bonded atoms) contribute to all terms. The relative weight of these contributions can then .be directly read from the V2 term. Since P, is nearly constant in the whole series (see Table III) the decrease of V2 in the order 2-phenylpyrimidine 5 4_phenylpyrimidine> !5-phenylpyrimidine is related to the corresponding increase of repulsions between non-bonded atoms. Table III.

PARAMETER q(1)

Mulliken populations from STO-3G wave functions for isomeric phenylpyrimidines. The atom numbering is that of Figure 1 and populations of hydrogen atoms are given in brackets near the populations of the carbon atoms to which they are bonded. The (Idensity matrix element for the inter-ring bond (Pn ) is also reported.

2-phenylpyrimidine

4-phenylpyrimidine

0.001

-0.003

5-phenylpyrimidine 0.003

q(2)

-0.057(0.084)

-0.053(0.087)

-0.063(0.065)

q(3)

-0.067(O.C62)

-0.063(0.067)

-0.059(0.067)

q(4)

-0.060(0.062)

-0.058(0.067)

-0.060(0.067)

q(5)

-0.067(0.062)

-0.063(0.065)

-0.059(0.067)

q(6)

-0.057(0.084)

-0.061(O.C64)

-0.063(0.065)

q(l')

-0.248

-0.244

-0.236

q(2')

0.195

q(3')

-0.248

q(4’ 1 q(5’ ) q(6’ 1 2Pn

0.049(0.084)

0.130(0.083)

0.124(0.082)

-0.250

-0.236

0.112

0.043(0.065)

-0.088(0.076)

-0.088(0.077)

0.049(0.084)

0.047(0.084)

0.043(0.065)

0.234

0.239

0.235

-0.020

23 Suitable functional forms for describing electrostatic and steric interactions are well known, so that our results open the possibility of deriving reliable torsional potentials for the class of azabiphenyls. The more so as the atomic charges of phenylpyrimidines are essentially constant (see 13,22 Table III) and very similar to those of benzene and pyrimidine.

REFERENCES /l/ J.W.Emsley. D.S.Stephenson, J.C.Lindon, L.Lunazzi and S.Pulga J.CHEM.SOC.(Perkin II) p-154 (1975 /2/ L.A.Carreira and T.G.Towns J.MOL.STRUCT. c.1

(1977).

/3/ F.Lelj, N.Russo and C.Chidichimo CHEM.PHYS.LETT. 69,530 (1980). /4/ L.Fernholt, C.Romming and S.Samdal ACTA CHKM. SCAND. A35.707 (1981). /5/ P.Freunrllich,E.Jakusek,K.A.Kolodziej,A.Koll,M.Poj~ovska and S.Sorriso J.PHYS.CHEM.g.1034(1983) /6/ G.Favini GAZZETTA 94.1287 (1964). /7/ V.Galasso,G.De Alti and A.AiRotto TETRAHEDRON 3.991

(1971).

1918

V. BARONEet al.

/8/ D.Perahia and A.Pullman CHEM.PHYS.LETT. 19,73 (1973). /9/ R.Benedix,P.Birner,F.Birnstock.H.Hennig and H.J.Hofmann J.MOL.STRUCT. =,99

(1979).

/lo/ W.J.Wells,H.H.Jaffe,N.Kondo and J.E.Mark MAKROMOL.CHEM. 183,801 (1982). /ll/ F.Momicchi.oli,I.Baraldi and M.C.Bruni CHEM.PHYS. 70,161 (1982). /12/ P.Birner and H.J.Hofmann INT.J.QUANT?JM CHEM. 2,833 /13/ V.Barone.F.Lelj,C.Cauletti.M.N.Piancastelli

(1982).

and N.Russo MOL.PHYS. 9,599

(1983).

/14/ V.Be.rone,C.Cauletti,F.Lelj,H.N.Piancastelli and N.Russo J.AM.CHEM.SOC. 104.4571 (1982). /15/ V.Galasso,L.Klasinc,A.Sablijic,N.Trinjastic,G.C.Pappalardo II) p.127 (1981).

and W.Steglich J.CHEM.SOC. (Perkin

/16/ E.Orti,J.Sanchez-Marin and F.Tomas THECCHEM 108,199 (1984). /17/ V.Barone,N.Bianchi,F.Lelj,N.Russo and G.Abbate THEOCHEM =,35

(1984).

/la/ S.Almlof CHEM.PHYS. 5,135 (1974). /19/ V.Barone,C.Cauletti,L.Commisso,F.Lelj,M.N.Piancastelli /20/ V.Barone,F.Lelj,L.Commisso,N.Russo,C.Cauletti

and N.Russo J.CHEM.RES.S.p.338 (1984).

and M.N.Piancastelli CHEM.PHYS., in press.

/21/ J.S.Binkley,R.A.Yhiteside,R.Krishnan,R.Seeger,D.J.DeFrees,H.B.Schlegel,S.Topiol,L.R.Kahn J.A.Pople QCPE 406 (1982) /22/ P.Pulay,G.Fogarasi,F.Pang and J.E.Boggs J.AM.CHEM.SOC. =,4832

and

(1979) and refs.therein

/23/ U.Burkert and N.L.Allinger MOLECULAR MECHANICS, ACS Monograph 177, American Chemical Society, Washington D.C. (1982) /24/ I.Fischer-Hjalmars TETRAHEDRON 19.1805 (1963). /25/ D.E.Willi.ams and R.R.Weller J.AM.CHEM.SOC. 105.4143 (1983). /26/ W.R.Busing J.AM.CHEM.SOC. a,4829

(1982).