The SPCl2F- radical : an ESR and ab initio quantum chemical study

The SPCl2F- radical : an ESR and ab initio quantum chemical

study

Citation for published version (APA):

Janssen, R. A. J., & Buck, H. M. (1986). The SPCl2F- radical : an ESR and ab initio quantum chemical study. Chemical Physics Letters, 132(4-5), 459-463. https://doi.org/10.1016/0009-2614%2886%2980646-7,

https://doi.org/10.1016/0009-2614(86)80646-7

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10.1016/0009-2614%2886%2980646-7 10.1016/0009-2614(86)80646-7

Document status and date: Published: 01/01/1986

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Volume 132, number 4,5 CHEMICAL PHYSICS LETTERS 19 December 1986

THE SWI,F- PHOSPHORS RADICAL.

AN ESR AND AB INITIO QUA~U~ CHEMICAL STUDY

Rent A.J. JANSSEN and Henk M. BUCK

Department of Organic Chemistry, Eindhoven University of Technology, 5600 MB Eindhoven. The Netherlands

Received 4 September 1986; in final form 7 Oktobet 1986

The SPCl,F- ph~p~o~nyl radical, obtained by X-irradiation at 77 K of WC&F, possesses a deformed trigonal bipyramidaI structure in which the two chlorine nuclei occupy the apical sites. The radical is identified by electron spin resonance. An ab initio calculation of the molecular geometry, the isotropic and dipolar hyperfine interactions, and the nuclear quadrupole interactions is presented.

1. Introduction

Phosphomnyl radicals (P&) can adopt a variety of electronic configurations and geometric structures [ 1,2]. Frequently a tri8onal bipyramidal structure with the unpaired electron in an equatorial position (TBP-e) is observed. Single crystal electron spin res- onance (ESR) [ 3,4] and quantum chemical calcu- lations [5,6] reveal that, as an approximation, the singly occupied molecular orbital (SOMO) of a TBP- e phosphoranyl radical is an antibonding orbital which contains large contributions from the phos- phorus 3s and 3p, orbitals and the valence np, orbit- als of the two apical ligands. The site preference of the substituent groups in a TBP-e phosphoranyl rad- ical is important for the spin density distribution when two or more different groups are present. In principle a substituent can be located either in an apical or an equatorial position. In general the most electronegative ligands are located on the apical site, consistent with the apicophilicities of five-coordi- nated phosphoranes [ 7,8].

An alternative structure for a phosphoranyl radi- cal is the cr* configuration in which the unpaired electron is located in an antibon~ng orbital between phosphors and one of the ligands. We have recently shown that if the tendency of one substituent to occupy an apical position is appreciably larger than for the other three a o* structure can be expected [ 91. We now report the ESR spectrum and an ab initio

Cl

t e-

/‘<F - _77K

Cl 1

Fig 1. Schematic representation of the SOMO of la.

quantum chemical calculation of the dichlorofluo- rophosphine sulfide radical anion ( SPCIZF- ) , la (fig.

1)) which represents a strongly deformed TBP-e type radical with two apical chlorine nuclei and an equa- torial ff uorine nucleus. The apparent violation of the ele~tronegativity rule is discussed. The quantum chemical calculations reveal that the determination of the radical geometry on basis of anisotropic hyperfine couplings is not always justified because the principal directions of the dipolar interactions are not necessarily coincident with the molecular frame.

A solution of SPClzF [ 101 in 2-methyltetrahydro- furan (MeTHF) was rapidly frozen in liquid nitro- gen. The sample was X-irradiated at 77 K using a Cu

anticathode for 6 h. ESR spectra were recorded digi- tally using a Bruker ER 200D spectrometer inter- faced with an Aspect 3000 computer. A sweep width of 0.1875 T was sampled with 4k data points result- ing in a resolution of 0.045 mT.

Quantum chemical calculations were performed using the GAUSSIAN 80 program [ 111 and the unrestricted Hartree-Fock (UHF) procedure. Iso- tropic and dipolar hyperfine interactions were eval- uated from the wavefunction after annihilation of the largest spin contaminant as described before [ 6,12 1. The 35Cl nuclear quadrnpole coupling constants were calculated as the product of the nuclear quad~pole moment Q and the largest value of the electric field gradient tensor q3 at the chlorine nuclei [ 13,141.

3. Results

The ESR spectrum of X-irradiated SPClrF in MeTHF at 100 K (fig. 2) shows clearly the features of a phosphoranyl radical formed by electron cap- ture. The high and low field components of the large 3’P doublet show an additional splitting resulting from two equivalent chlorine nuclei. Analysis of the spectrum in terms of anisotropic 31P and 35Cl/37Cl hyperfine couplings is not possible. The parallel 3sCl splittings are well defined but the pe~endicular fea- tures are not resolved. This is a frequently observed

Fig. 2. First derivative ESR spectrum at 100 K of X-irradiated SPC&F in MeTHF.

phenomenon in powder spectra of chlorine contain- ing radicals which results from the relatively large nuclear quadrupole interaction and second-order effects [ 15,16]. For radicals in which the direction of the principal hyperfine coupling coincides with the major quadrupole axis, the spectra yield a normal equally spaced multiplet when the magnetic field is parallel to this axis. For a perpendicular orientation of the magnetic field, however, complicated spectra can occur particularly when the quadrupole interac- tion is of the same order of magnitude as the hyper- fine energy [ f 7- 19 J. The important result that the two chlorine nuclei are equivalent, and bear a large amount of spin density, is not affected by the com- plexity of the spectrum. Furthermore no 19F hyper- tine coupling is observed and hence the spin density on fluorine will be small. The results lead to the assignment that the SPC12F- radical anion possesses a TBP-e like structure in which the two chlorine nuclei are located apically and the fluorine and sul- fur nuclei are in equatorial positions.

After correction according to the Breit-Rabi equa- tions the spectrum of SPCl*F- yields&= 2986 MHz,

A Cl = 167 MHz and a g value of 2.010. Assuming a

perpendicular orientation for the 31P and 35Cl hyper- tine tensors these values correspond to Al (=Ai,-B) for 3’PandAy( =A,,+2B) for 35Cl. The 35Cl/37Cl couplings of SPC12F- are comparable to other TBP-e phosphoranyl radicals in which two chlorine atoms occupy apical positions as O&l,

[4], PCL [20], and SPCl, [L!l].

A quantum chemical description of the S&&F- radical was obtained by using the UHF method at the SCF level with the standard 4-31G basis set [ 22,231 implemented with a single set of six second- order Gaussians on P, S and Cl (radical coefficients P 0.55, S 0.65 and Cl 0.75) [24]. Gradient optimi- zation without any symmetry constraint resulted in a structure of C, symmetry (fig. 3, E= - 1745.9538 au and (S2) =0.7506).

The P-Cl bond length in this structure is 2.34 A and the Cl-P-Cl angle is 134.1’. The dihedral angle of the ClPCl and SPF planes is exactly 90”. The SOMO is an antibonding orbitat with large contri- butions from the central phosphorus nucleus and the two chlorine substituents, in accordance with the experimental observation. The calculated isotropic and anisotropic hyperfine couplings of this radical 460

Volume 132, number 45 CHEMICAL PHYSICS LETTERS 19 December 1986 f :%I i f/ %

;

.HY

-_

?iL

----___

PC ____

4

Fig. 3. Optimized geometrical parameters for SPC12F- and directions of&,& andq,. BonddistancesPCl=2.32 A, PS= 1.92

A, PF=1.60 A; bond angles ClPCl=134.1”, ClPS=lll.O”, ClPF=91.4”, SPF= 109.8”. Cl, and CIZ in x,zplane: S and F in x,Y plane.

obtained after annihilation of the corresponding UHF wavefunction (table 1) are in accordance with the experimental data and give a further insight into the structure of this radical.

The directions of the chlorine anisotropic hyper- fine interactions are inclined by an angle of 158.3” and nearly perpendicular to the phosphorus dipolar interaction. The near alignment of the ligand hyper- fine couplings (usually chlorine or fluorine) has often been invoked to be an indication and a confirmation of a TBP-e structure [ 4,15,25]. Our calculations reveal that the Cl-P-Cl bond angle in S&&F- is only 134” and that the actual molecular geometry is in fact intermediate between a tetrahedral configuration and a TBP structure.

The computation of the electric field gradient ten- sor (table 2) at the position of the chlorine nuclei reveals an angle of only 2.7” between the direction of q3 and the P-Cl bond, and of 13.3” between q3 and the direction of the largest chlorine hyperfine coupling.

The calculated nuclear quadrupole coupling con- stant ( -4 MHz) is of the same order of magnitude and of similar direction as the chlorine hyperfine coupling.

4. Discussion

Both experiment and theoretical calculations reveal that the SPC12F- radical possesses an electronic

Table 1

Calculated isotropic and anisotropic hyperftne coupling constants for SPC12F-

Nucleus A,, E Direction cosines a1

(au) (MHz) (au) (MHz) x Y 2 P 1.423 2577 1.216 263 0.925 0.381 0.000 -0.565 - 122 0.381 -0.925 -0.000 -0.651 -141 - 0.000 - 0.000 1 .ooo Cl, 0.129 56 1.370 72 0.147 -0.118 0.982 -0.682 -36 0.869 0.849 -0.071 -0.688 -36 0.471 -0.864 -0.175 Cl* 0.129 56 1.370 72 0.147 -0.118 -0.982 -0.682 -36 0.869 0.489 0.07 1 -0.688 -36 0.471 -0.864 0.175 F 0.005 21 0.138 69 0.642 0.767 -0.059 -29 0.761 -0.641 -0.079 -40 -0.000 -0.001 S 0.001 0.542 0.963 0.271 -0.259 0.271 -0.970 -0.283 0.000 -0.001 0.001 -0.001 1.000 0.000 -0.001 1.000 a) Direction cosines refer to coordinate system in fig. 3.

Table 2

Electric field gradient tensor and nuclear quadrupole coupling (NQC) for SPC&F-

Cl, Clz 4 (au) -1.148 - 1.061 2.209 - 1.148 - 1.061 2.209 NQC (MHz) -41 -41 Direction cosines a) x Y -0.295 0.946 0.889 0.321 0.350 -0.019 -0.295 0.946 0.889 0.321 0.350 -0.019 z 0.129 -0.326 0.936 -0.129 0.326 -0.936 a) Direction cosines refer to coordinate system system in fig. 3.

structure in which the major part of the unpaired electron density is located on phosphorus and the two equivalent chlorine atoms. The chlorine nuclei occupy the axial positions of a strongly deformed TBP-e structure. The tendency of chlorine to con- tribute to the SOMO and to occupy an apical posi- tion thus appears to be superior to that of fluorine. This is in accordance with our recent results on the structure of chlorine and fluorine diaminophosphine sulfide electron capture radicals [ 91. Five-coordi- nated phosphoranes show an opposite behavior for the relative apicophilicity of fluorine and chlorine since PF$& is known to have both chlorines in equatorial position [ 261. Although, in general, TBP- e phosphoranyl radical structures agree well with the principles of five-coordination an important differ- ence is the nature of the highest occupied molecular orbital. For a phosphoranyl radical this orbital is half- filled and highly antibonding between phosphorus and the two p-orbitals of the apical ligands. Ab initio calculations for TBP-e phosphoranes reveal that the highest symmetrical MO is essentially non-bonding or slightly antibonding between the central atom and the apical nuclei [ 27-291. On this basis the apical bonds in a phosphoranyl radical can be expected to be longer and weaker than the corresponding bonds in a phosphorane. Upon formation of a TBP-e struc- ture after electron capture of a four-coordinated compound the site preference of the substituents will partly depend on the intrinsic P-substituent bond energy. The P-Cl bond energy is known to be approximately 170 kJ mol-’ smaller than the P-F bond energy, explaining the observed axial location of chlorine.

462

Acknowledgement

This investigation has been supported by the Netherlands Foundation for Chemical Research (SON) with financial aid from the Netherlands Organization for the Advancement of Pure Research (ZWO).

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