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Molecular structure of biphenyl in an argon matrix
Volume 46, number
3
CHEMICALPtIYSlCS LITIXRS
MOLECULARSTRUCTUREOFBIPHENYLIN
AN
15 March 1977
ARGONMATRiX
Luc LE GALL Laboratoirtz de Thermoriynamique 2928EBrcst
Chimique, FacultG des Scimcer de Brat,
Cedcx, France
and Shigeko SUZUKI Department o/Chemistry SaIford MS 4 WT. UK
Received 20 December
and Applied
Chcmlstry,
University of Salford,
1976
The infrared spectrum of biphenyl wag measured m an Ar matrk at 20 K. A comparison of the results with the spectra of this moiecule in the sohd and liquid states shows that the structure of biphcnyl III the matrix is simdar to that in the hquid state, that is, the moIeculc i%non-pLmar.
The coupling between the two benzene rings in biphenyI is known to be weak, and its dihedrai angle 0 about the central C-C bond varies according to its physical states, i.e. 19= O” (planar) in the solid state [ 1J , 8 = I9”-26” in the liquid state [2] , and 0 = 40’ in the gaseous state [2,3]. The degree of coplanarity perturbs the electronic distribution in the molecule, because the extent of delocalisation is affected and, in turn, the intensity of infrared and Raman wbrational bands is changed. The intensity changes in the infrared bands of in-plane vibrations in biphcnyl were analysed in det-di in going from solid to liquid by Sournia [4] _ He has observed that the band intensities of the B3,, modes increase, whiie the B,, modes decrease in intensity when biphenyl undergoes the phase transition from solid to liquid. Men two benzene rings (Dsh) couple to form a biphenyl molecule, a degenerate, in-plane mode of bcnzcne (E2 or El,) gives rise to four non-degenerate modes o Hbiphenyl, Ag, B3,,, Bls, and B,, of which only the E,, and B,, vibrations are infrared active. Since the coupling between two ben?cne rings is weak, the frequencies of these “paired” bands (B3,, and B2,,) are close and they arc easily identified. For exampIe, the v8 (&) band of benzene at 1595 cm-l appears
in biphenyl. Thus it is more practical to measure ttrc reIativc intensity of these “paired” bands in the same state, for example &Jlsb, rather than to compare directly the intensities of the same mode in different states, fur cxample /8,(liquid)/f8,(solid). Sournia measured the rcIative intensity of “paired” bands in the solid state and in the liquid state *, and found that in the solid sEate, was less than 0.3, while in the liquid state the IB I’B va?& wz:grcdter than 1.O (table 15 of ref. [elf_ This intensity change can be explained simply as follows: the B,, mode intensity is expected to be affected when the dihedral angle varies, since is the IQ, modes the transition moments (in-plane) of two benzene rings lie m the direction perpendicular to the ccntre C-C axis. On the other hand, the B,, mode intensity shouId be less affected by a change in dihedral angle, because in this species the transition moments of two benzene rings are parallel to the centre C-C bond. in his more detailed analysis, Sournia calculated the intensity variation using IL CNDO programme, and showed that this simple scheme worked at least qualitatively, thus proving that the inten* tie also measured the intensity ratio of the same band in different
physical states.
467
Volumc46, number
CHEMICAL PHYSICS LETTERS
3
L_
----
_-__>_
---
IIIPIII-'sj L SC-l.1 D (T
_ A:?
1
i)
15 March 1977
matrix even at a temperature well below the melting point, because the intermolecular forces are absent. We measured the infrared spectra of biphenyl in an isotropic Ar matrix at 20 K. It is difficult to know the exact concentration of our matrices, since biphenyl was deposited from the solid state by evaporation. However, an examination of the concentration dependence of the spectra shows that the bands due to dimcr arc undetectably weak. Experimental details about making matrices from solid samples arc given elsewhere [S] . The spectrum of biphenyl in an Ar matrix at 20 K is shown rn fig. 1, together with the spectra of solid and liquid biphcnyl taken from ref. [4]. It is immediately obvious that there is a striking resemblance between the liquid and matrix spectra.
------l
I----“--
l---
_-__
-_-_-L--L
-
-
-
1
---I
L
The frequency shifts observed are due to the matrix cffds. 1i-1fact, when we cxdmine the spectrum in more detail, WCfind that the intensity ratio of B3,,-B2” pair bands is greater than one when measurable, i.e. for the pairs 9a/9b, 8a/8b, 19a/19b, and 18a/18b, in accordance with the liquid spectrum. Slight differences arc observed in vsa whcrc the band splits and in uI~JvI~~ pair where the band shape is similar to that of tfre high temperature solid (G5”C) rather than the liquid. However, in general, the matrix spectrum is very similar to the liquid spectrum, thus suggesting a nonplanar structure in an Ar matrix. In conclusion, the dihedral angle of biphenyl in an Ar matrix IS likely to be Ig”-260, that is, i?s structure is similar to that in the liquid state.
IGg. 1.
sity change in infrared bands of biphenyl observed on going from solid to liquid was due to a change in the dihedral angle. It is an interesting problem to know the molecular structure of biphcnyl in a matrix at low temperature. If the intermolecular forces are responsible for the biphenyl molecule being planar in the solid state, it is expected that the structure will be non-planar in a
468
References A. Hargreavcs and S.H. Rizvi. Acta Cryst. 15 (1962) 365. II. Suzuki, Bull. Chcm. Sot. Japan 32 (1959) 1340. G.H. Bcaven, in: Stcric effects in wnjugatcd systems, cd. G.W. Gray (Butterworths, London, 1958). A. Sourma, Thh, Pcrpignan (1972). A.J. Barnes, L. Ic Gall, C. hdec and J. Lauransan, J. Mol. Struct., to bc published.
3
CHEMICALPtIYSlCS LITIXRS
MOLECULARSTRUCTUREOFBIPHENYLIN
AN
15 March 1977
ARGONMATRiX
Luc LE GALL Laboratoirtz de Thermoriynamique 2928EBrcst
Chimique, FacultG des Scimcer de Brat,
Cedcx, France
and Shigeko SUZUKI Department o/Chemistry SaIford MS 4 WT. UK
Received 20 December
and Applied
Chcmlstry,
University of Salford,
1976
The infrared spectrum of biphenyl wag measured m an Ar matrk at 20 K. A comparison of the results with the spectra of this moiecule in the sohd and liquid states shows that the structure of biphcnyl III the matrix is simdar to that in the hquid state, that is, the moIeculc i%non-pLmar.
The coupling between the two benzene rings in biphenyI is known to be weak, and its dihedrai angle 0 about the central C-C bond varies according to its physical states, i.e. 19= O” (planar) in the solid state [ 1J , 8 = I9”-26” in the liquid state [2] , and 0 = 40’ in the gaseous state [2,3]. The degree of coplanarity perturbs the electronic distribution in the molecule, because the extent of delocalisation is affected and, in turn, the intensity of infrared and Raman wbrational bands is changed. The intensity changes in the infrared bands of in-plane vibrations in biphcnyl were analysed in det-di in going from solid to liquid by Sournia [4] _ He has observed that the band intensities of the B3,, modes increase, whiie the B,, modes decrease in intensity when biphenyl undergoes the phase transition from solid to liquid. Men two benzene rings (Dsh) couple to form a biphenyl molecule, a degenerate, in-plane mode of bcnzcne (E2 or El,) gives rise to four non-degenerate modes o Hbiphenyl, Ag, B3,,, Bls, and B,, of which only the E,, and B,, vibrations are infrared active. Since the coupling between two ben?cne rings is weak, the frequencies of these “paired” bands (B3,, and B2,,) are close and they arc easily identified. For exampIe, the v8 (&) band of benzene at 1595 cm-l appears
in biphenyl. Thus it is more practical to measure ttrc reIativc intensity of these “paired” bands in the same state, for example &Jlsb, rather than to compare directly the intensities of the same mode in different states, fur cxample /8,(liquid)/f8,(solid). Sournia measured the rcIative intensity of “paired” bands in the solid state and in the liquid state *, and found that in the solid sEate, was less than 0.3, while in the liquid state the IB I’B va?& wz:grcdter than 1.O (table 15 of ref. [elf_ This intensity change can be explained simply as follows: the B,, mode intensity is expected to be affected when the dihedral angle varies, since is the IQ, modes the transition moments (in-plane) of two benzene rings lie m the direction perpendicular to the ccntre C-C axis. On the other hand, the B,, mode intensity shouId be less affected by a change in dihedral angle, because in this species the transition moments of two benzene rings are parallel to the centre C-C bond. in his more detailed analysis, Sournia calculated the intensity variation using IL CNDO programme, and showed that this simple scheme worked at least qualitatively, thus proving that the inten* tie also measured the intensity ratio of the same band in different
physical states.
467
Volumc46, number
CHEMICAL PHYSICS LETTERS
3
L_
----
_-__>_
---
IIIPIII-'sj L SC-l.1 D (T
_ A:?
1
i)
15 March 1977
matrix even at a temperature well below the melting point, because the intermolecular forces are absent. We measured the infrared spectra of biphenyl in an isotropic Ar matrix at 20 K. It is difficult to know the exact concentration of our matrices, since biphenyl was deposited from the solid state by evaporation. However, an examination of the concentration dependence of the spectra shows that the bands due to dimcr arc undetectably weak. Experimental details about making matrices from solid samples arc given elsewhere [S] . The spectrum of biphenyl in an Ar matrix at 20 K is shown rn fig. 1, together with the spectra of solid and liquid biphcnyl taken from ref. [4]. It is immediately obvious that there is a striking resemblance between the liquid and matrix spectra.
------l
I----“--
l---
_-__
-_-_-L--L
-
-
-
1
---I
L
The frequency shifts observed are due to the matrix cffds. 1i-1fact, when we cxdmine the spectrum in more detail, WCfind that the intensity ratio of B3,,-B2” pair bands is greater than one when measurable, i.e. for the pairs 9a/9b, 8a/8b, 19a/19b, and 18a/18b, in accordance with the liquid spectrum. Slight differences arc observed in vsa whcrc the band splits and in uI~JvI~~ pair where the band shape is similar to that of tfre high temperature solid (G5”C) rather than the liquid. However, in general, the matrix spectrum is very similar to the liquid spectrum, thus suggesting a nonplanar structure in an Ar matrix. In conclusion, the dihedral angle of biphenyl in an Ar matrix IS likely to be Ig”-260, that is, i?s structure is similar to that in the liquid state.
IGg. 1.
sity change in infrared bands of biphenyl observed on going from solid to liquid was due to a change in the dihedral angle. It is an interesting problem to know the molecular structure of biphcnyl in a matrix at low temperature. If the intermolecular forces are responsible for the biphenyl molecule being planar in the solid state, it is expected that the structure will be non-planar in a
468
References A. Hargreavcs and S.H. Rizvi. Acta Cryst. 15 (1962) 365. II. Suzuki, Bull. Chcm. Sot. Japan 32 (1959) 1340. G.H. Bcaven, in: Stcric effects in wnjugatcd systems, cd. G.W. Gray (Butterworths, London, 1958). A. Sourma, Thh, Pcrpignan (1972). A.J. Barnes, L. Ic Gall, C. hdec and J. Lauransan, J. Mol. Struct., to bc published.