Electronic structure and conformational properties of vinylphosphonic

Jan 1, 1994 - J. A. Dobado, Henar Martínez-García, José Molina Molina, and Markku R. Sundberg. Journal of the American Chemical Society 1998 120 (33),...
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J. Phys. Chem. 1994,98, 1109-1 116

1109

Electronic Structure and Conformational Properties of Vinylphosphonic Acid and Some Related Derivatives A. Hern6ndez-Laguna,**t*L C. I. Sainz-Diaz,*J Y. G. Smeyers,? J. L. G. de Paz,* and E. G6lvez-Ruanos Instituto de Estructura de la Materia (CSIC), CISerrano 123, 28006 Madrid, Spain; Departamento de Quimica- Fisica Aplicada, C-XIV-505, Uniuersidad Autbnoma de Madrid, 28049 Madrid, Spain; Departamento de Quimica Orghnica, Uniuersidad de Alcalh de Henares, Madrid, Spain; and Estacibn Experimental del Zaidin (CSIC), ClProfesor Albareda I , 18008 Granada, Spain Received: March 19, 1993”

The electronic structure and conformational properties of vinylphosphonic acid, vinylphosphoryl dichloride, and vinylphosphine oxide have been studied by ab initio quantum mechanical methods. Furthermore, the vinylphosphonic acid has been studied experimentally by means of 13CN M R and ultraviolet spectroscopy. This compound has been synthesized by a modification of the McKenna hydrolysis reaction, improving significantly the yield. T h e ab initio calculations have been performed with different basis sets, including correlation energy a t the MP2/6-31G* level. Topological analysis of charge density has also been carried out. Two conformers, s-cis and s-trans-gauche,have been determined for the internal rotation around the C-P bond, with low rotational barriers. Two transition structures, s-cis-gauche and s-trans, have been located. A partially polarized strong triple bond for the phosphoryl group has been found. A weak ?r conjugation has been detected in the C=C/ P=O system. T h e chemical shifts and absorption coefficients determined experimentally are in agreement with the electronic structure and conformational properties calculated by ab initio methods.

1. Introduction

Therefore, the planar configurations should be expected to be a stable conformer. However, the trans conformation is not found Phosphonates have become very interesting compounds used to be a minimum but a transition structure.Il The nonplanar as Wittig-Horner-Emmons’ reactives, in cycloaddition reactions, conformers could be accounted for by means of a p,-d, conjugative drugs (fosfomycin,l-3 fo~midomycin~) and herbicides,5 polymer effect undergone16 by the tetrahedral electronic structure of the additives,6 flame retards,’ and metal extractants.8 phosphorus atom. Ishmaeva” studied the structures of the In previous papers, alkenylphosphonates and alkenylphosphonic organophosphorus compounds by dipolar moment techniques. acids were studied by means of N M R and IR spectra and ab The author concluded that the relative stabilities of the different initio calculations at the STO-4G minimal basis set l e ~ e l . ~ - l ~ conformers are controlled by polar factors, and the ?r-electron From the propenylphosphonate’s N M R spectra, the conformers moiety of the molecule has hardly any influence on the conforseem to present a planar conformation, but it is not clear whether mations of the irregular groups linked to the phosphorus atom. they are in s-cis or s-trans configurations. The vinylphosphonic A complete knowledge of the electronic structure and conacid was partially studied a t STO-4G and 3-21G levels.I1 formational properties of the phosphonic derivatives should be Related compounds, such as the phenylphosphoryl dichloride, significant because of their numerous applications. This is the have been studied by means of electronic diffraction in gas phase. main goal of this paper on vinylphosphonic acid (VPA), The most probable conformation, with the P=O bond coplanar vinylphosphoryl dichloride (VPDC), and vinylphosphine oxide in the phenyl plane, has been found.’* Applying the same (VPO). An analysis of the topology of the charge density is technique on the vinylphosphoryl dichloride, a cis coplanar (or applied to the possibility of a C=C/P=O conjugative effect and quasi cis) C=C/P=O configuration has been found as the most the character of the phosphoryl bond. An ab initio study on the stable conformer.13 However, in the complex of 0,O-diethylelectronic and conformational properties on the three compounds phenylphosphonite with different metals, a noncoplanar phenyl/ is also presented as a function of the basis length. P=O configuration (e = 5 5 - 7 5 O ) has been detected.14 These compounds were chosen because they exhibit the main Ewig et al. have studied the electronic structure of phosphonic conformational and electronic features of the vinylphosphonic acid by means of quantum chemical calculation^.^^ These authors system. The present study is a first step for researching the determined the most stable conformations using STO-3G*, electronic structure and conformational properties of more 3-2 1G*, and. 4-3 1G* basis sets. complex alkenylphosphonic derivatives. Kabachnik pointed out that the j3-oriented nucleophilicreagent properties of the vinylphosphonic group induce the idea that it 2. Introduction to the Charge Density Topology Theory could exist with a certain conjugation between the vinylic moiety The topological properties of the charge density p(r) in a bond and thephosphoryl group.16 However, the spectroscopic properties path (ref 18 and references inside) are a way to study the electronic do not show any conjugative effect. The author concluded that nature of the bonds. The topology of a function is characterized such a weak conjugative effect exists, but it is not strong enough by its critical points. These are obtained in the point io where to be detected in the spectroscopic measurements but is large Vp(r,) = 0, and are classified according to the three canonical enough to be observed in the reactive properties.16 curvatures, Xi, of the Hessian matrix (Hij = a2p(r)/aiaj,with i, At first glance, the planar configurations could be explained j = x, y , 2 ) . The critical points are characterized by their rank by a conjugative effect between the C = C and P=O bond system. and signature (ra,s). The rank is the number of non-null t Instituto de Estructura de la Materia (CSIC). eigenvalues, and the signature is the algebraic addition of the f Universidad Aut6noma de Madrid. positive and negative eigenvalues. In this paper, the studied 8 Universidad de AlcalL de Henares. molecules yield two kind of critical points: the (3,-3) and (3,-1). 1 Estaci6n Experimental del Zaidln. .a Abstract published in Advance ACS Abstracts, December 1, 1993. The first is a maximum of the charge density in the three spatial 0022-365419412098-1109%04.50/0 0 1994 American Chemical Society

1110 The Journal of Physical Chemistry, Vol. 98, No. 4, 1994 coordinates and physically corresponds t e t h c u d e i . The second has two negative curvatures and a positive one. This is a saddle point of p(r), which is a maximum with respect to two spatial coordinates and a minimum in the third one, and corresponds to a bond critical point. The positive curvature, X3, is associated with the trajectory where the charge density is always maximum and links two nuclei; this is called bond path. The two negative curvatures, A1 and h2, are associated with perpendicular directions to the bond path. The ellipticity of a bond is defined by means ofthese twolastcurvatures: E = X 1 / X 2 - l,whereX2is thesmallest one. A bond with cylindrical symmetry yields a null ellipticity. The topology of p(r) does not define directly the type of bond: single, double, or triple. The ellipticity of a single bond is zero because of its cylindrical symmetry. However, the double bond, owing to its 7~ character, has the charge density accumulated in a given plane so that both curvatures are different, giving an ellipticity >> 0. In the triple bond, the second r orbital is filled and the bond again reaches the axial symmetry.I9 V2p(rc)is also valuable to know and classify the nature of the atomic interactiom20 This magnitude is the algebraic addition of the three curvatures (V2p(rc) = z;Xi). In the covalent bonds V2p(rc) < 0 (the two negative eigenvalues dominate), and in intermediates and closed-shell interactions (ionic, van der Waals, and hydrogen bonds) V2p(rc) > 0 (the positive curvature dominates).20 In an ionic bond, the more positive is V2p(rc)the smaller is p(rc). The V2p(rc)is related to the potential (V) and kinetic (C) energy densities by

-L(r) = (h2/2m)V2p(r) = V(r) + 2G(r) The kinetic energy density C(r) is, by its definition, everywhere positive, and V(r) is observed to be everywhere negative.20 In a bond critical point (3,-l), the positive curvature is dominant. As a consequence of the contraction of the charge in the direction of the nuclei, the kinetic energy density is large. The ratio C(rc)/ p(rc) is also used to classify the atomic interactions. This ratio is lower or greater than unity in the covalent or ionic interaction, respectively.

3. Methods 3.1. Theoretical Methods. The ab inirio calculations have been performed with the MONSTERGAUSS,21 GAUSSIAN 80,22and GAUSSIAN 8623programs. Full geometry optimizations were performed with no symmetry restrictions using different gradient method^.^^-^^ The VA05D29subprogram has been used to locate the transition states of the potential energy hypersurface (PEH). This method yields a saddle point of any order, and the user does not need to declare it. These structures were characterized by performing canonical analysis, calculating the Hessian by means of analytical first derivatives and numeric second derivatives. The calculations were carried out to reach average gradients less than or equal to 5 X lo4 mdyn or mdyn A/rad. The procedure for locating transition structures resorted to a partition of the internal coordinate space into two subspaces: the so-called control space30*31 and the complementary space. The first is defined by all the coordinates whose forces change meaningfully when the reaction coordinate changes. The complementary subspace is composed of the remaining internal coordinates. In this case, the reaction coordinates are the internal rotations angles. After each critical point is located with only a negative canonical curvature on the surface, the complementary space is minimized in separate runs, following an alternative procedure on the control and the complementary spaces. This procedure is repeated to find minimum values for the gradient vector components. In order to locate the actual conformers on the PEH, the whole internal coordinate space was finally used.

Hernbndez-Laguna et al.

"

I

(0)

&OH

fbl

do

'0

0

'H

id

+e,

90 s-cis

anti

The GAUSSIAN 92 program32has been used to optimize all the critical points at the 6-3 l G * level with electronic correlations by second-order Moller-Plesset perturbation^^^ using a frozen core.34 The Boys method was used toobtain localized molecular orbitals (LMO).~~ The SPARTAN program36was used to calculate and draw the molecular electrostatic potential (MEP) on the electronic surface of 0.002 e/A3. The red and blue colors stand for the most negative and most positive values of the potential, respectively, in Figure 3. The topological properties of the charge density have been performed on the conformational minimum of vinylphosphonic acid. In order to study the bonding in the phosphoryl group, a certain number of related molecules have been also studied. The KGNMOL program was used to calculate the critical points of the charge density.37 3.2. Experimental Methods. The ultraviolet spectroscopical studies were performed using a Perkin-Elmer-Coleman-570 double-beam spectrophotometer. The allylphosphonic acid has been synthesized by the Arbuzov reaction between 3-bromo- 1-propene and trimethylphosphite with a posterior acid h y d r ~ l y s i s ;X,,,(MeOH) ~~,~~ = 203 nm (e = 30). The vinylphosphonic acid (VPA) was has been synthesized by hydrolysis of diethyl vinylphosphonate,4° with the method described by McKenna et aL4' In this paper, this method has been modified by using HCl/HIO directly instead of C1SiMe3,42 increasing the yield up to 90% of vinylphosphonic acid; A,,(MeOH) = 204 nm (c = 179). I3C-NMR studies on the vinylphosphonic acid (VPA) have been also carried out and were recorded at 50.3 MHz with a Varian VXR 200 spectrometer, using DSS as an internal reference. I3C-NMR (D20) 6 (ppm): 136.7 (t, IJ(CH) = 166 Hz, C=CP), 129.6 (dt, 'J(CH) = 166 Hz, lJ(CP) = 177 Hz, C=C-P).42 4. Results and Discussion

The three conformational angles (el, 82, and 83; see Figure 1) and the two bending angles ( C I - C ~ - P ~and C~-P3-04) have been considered as the control space in the vinylphosphonic acid. The conformation with P=O and C=C bonds in the cis position is the origin of the rotation angle O1 ( O O , it is a s-cis conformer); the origin of rotation angles 82 and 8 3 is defined by the 0-H bonds in the syn position with respect to the P=O bond (Figure 1). 4.1. Conformational Properties. In Table I, the main geometrical features of the critical points of the potential energy

The Journal of Physical Chemistry, Vol. 98, No. 4, 1994 1111

Structure and Properties of Vinylphosphonic Acid

TABLE I: Conformational Angles, Energy, and Dipolar Moment of tbe Critical Points of the Potential Energy Hypersurface of VPA, VPDC, and VPOs**at Different Basis Sets compd baseC M1 TSl M2 TS2 VPA

6-31G'

el e2 e3

AEd

P

6-31G**

el e2 e3

AEd

P

MP2/6-31G'

01

e2 e3 AEd

P

VPDC

6-31G*

81 AEd P

MP2/6-31G*

81 AEd P

VPO

6-31G*

el

AEd

P

MP2/6-31G*

81 AEd P

0 -11.8 12.0 0.0 1.01 0 -9.7 10.1 0.0 1.07 0 -15.9 16.3 0.0 0.94 0 0.0 4.2 0 0.0 4.27 0 0.0 4.19 0.0 0.0 4.28

60.7 -10.8 20.5 2.7 61.2 -9.8 20.6 2.8 58.4 -7.5 30.6 2.3 71 4.3 63.2 3.3 64.9 3.5 59.8 2.76

115 -17.2 5.5 0.7 1.76 119 -19.8 2.2 0.8 1.87 116.8 -30.7 -0.9 0.4 1.97 26.5 2.9 4.7 17.4 2.1 4.79 23.3 1.42 4.84 117.2 0.74 5.00

# \

180 -13.6 13.4 3.2 180 -12.9 13.1 3.1 177.9 -59.4 21.8 2.3 80 3.6 80.0 3.2 80 3.17 180.0 3.07

M1, the most stbale conformer;M2, secondary conformer; TS1 and TS2, transition states. 81, C2C1PO4; 02, H705P04; 03, HsOsPO4. The

geometries have been optimized at the same basis set in each case. kcal mol. hypersurface (PEH) of the three compounds are listed, using different basis sets (6-31G* and 6-31G**). Preliminary calculations at STO-3G* and 3-21G* levels were performed but are not reported since they are essentially identical to those found at the 6-31G* level. In total, four critical points have been located: two minima and two transition states. For the vinylphosphonic acid, the most stable conformer ( M l ) corresponds to a s-cis conformation (01 = Oo) throughout all the basis sets. The calculations yield secondary conformers about 117O in all basis sets, and the difference in energies between both conformers is 0.8 kcal/mol for the 6-3 1G** basis set, as expected,43whenever the interactions depend mainly on the steric and polar effects. The electronic correlation was taken into account by means of second-order Moller-Plesset calculation^^^ at the 6-3 l G * basis set. This calculated correction yields qualitatively the same conformational results. The rotational barriers and the differences of energy between the conformers are lower than the calculations without correlation energy, although the rotational barrier of TS2 decreases more deeply than that of TS1. The transition states coincide with the s-trans or s-cis-gauche conformations, approximately 01 = 180' or 60°, respectively. Both of them have similar rotational barriers, increasing with the extension of the basis set and the addition of d functions. With the best calculations, MP2/6-3 1G*, two equal rotational barriers of 2.3 kcal/mol are found. If the C=C/P=O conjugative effect were strong enough, these rotational barriers should be greater. Notice that the s-trans conformation (01 = 180O) is definitively not a minimum, as might be thought at first glance, because of its similarity with the s-trans conformation of acrylic acid, as well as the posible conjugation effect between the C=C and P=O bonds.16 Rather, this configuration is a transition state. In a planar conjugated molecule, such as acrylic acid, the conjugative effect is PA-PA. In vinylphosphonic acid, the nonplanar conformers could be explained by a possible P A - ~ A

\

H

Figure 2. Configuration C=C/P=O s-trans of the vinylphosphonic acid

(Newman projections). conjugation. However, the overlap between the d r and PA orbitals is meaningless." These conformational properties could be due mainly to polar1' and repulsion effects. For instance, the two OH groups in the trans transition state adopt a configuration in which the two lone electronic pairs are approximately pointing to the C=C double-bond A cloud (see Figure 2). This configuration would yield a maximum of electronic repulsion which should transform the trans conformation in a transition state. The OH groups in the 0-H/P=O system show a syn conformation in most structures. This syn conformation could be accounted for with a maximal dipole-dipole interaction 0-H/ P=O. In vinylphosphoryl dichloride (VPDC), the M 1 (minimum) conformer coincides with the s-cis configuration, and M2 is s-trans-gauche located at 117' (without electronic correlation corrections this angle is larger). The difference in energies between both conformers is larger than that in VPA, as could be expected from the greater van der Waals radii of the chlorine atoms. The prediction of the s-cis coplanarity conformers is in agreement with the gas-phase experimental data from electronic diffraction spectro~copy.'~ The angles of the two transition states show values similar to those of vinylphosphonic acid, but the rotational barriers are higher than those of VPA. The conformational properties of vinylphosphine oxide (VPO) are quite similar to those of VPA and VPDC. The more stable conformer is the s-cis, and the secondary one corresponds to an angle close to 117O. At MP2/6-31G* basis set, the energetic difference between both minima is 0.7 kcal/mol, higher than in VPA. Both rotational barriers are different. The highest barrier is located on the s-trans conformation (0, = 180O). The rotational barriers of the compounds VPA, VPDC, and VPO are found to be low indeed; therefore, it can be deduced that the C=C/P=O conjugative effect should be very small. It is interesting to compare the present C=C/p--O compounds with the previously studied C=C/C=O compounds. Loncharich et determined the electronic structures and conformational properties of acrolein, acrylic acid, and methylacrylate by means of ab initio calculations at the 3-21G and 6-31G* levels. They found the most stable conformer in the C=C/C=O s-cis and C=O/O-H syn configurations. In these systems, the s-trans configuration was found to be a secondary conformer instead of a TS. 4.2. Molecular Geometry. In Table 11, the experimental geometrical datal3 and the ab initio calculations using different basis sets for the VPDC have been presented. The C=C bond is longer using the MP2/6-31G* than the other calculations, being very close to the experimental value. The root mean square (rms) deviations with respect to the experimental values for the main bond lengths show that 6-3 lG* yields, indeed, the best agreement with experimental data, especially with the more polarized bonds (P=O and P-Cl). These bonds are best described a t the 6-3 l G * level. Therms deviations for the main bonds angles

Hernbndez-Laguna et al.

1112 The Journal of Physical Chemistry, Vol. 98, No. 4, 1994 TABLE Ik Main Geometrical Features' of Minimal Conformer of VPDC Determined by ab Initio Calculations at Different Basis Sets and Experimental Geometrical Data parameter C=C c-P P=O P-CI C2-H c=c-P C-P=O CI-P-CI

ci-p=o

experimental

STO-3G*

6-3 lG*

MP2/6-3 lG*

1.338 1.817 1.455 2.016 1.080 118.2 119.5 101.6 116.8

1.314 1.794 1.440 2.001 1.085 119.9 116.9 100.7 115.4 0.018 0.9

1.321 1.784 1.449 2.018 1.075 119.0 116.2 102.3 113.8 0.017 2.3

1.340 1.785 1.484 2.029 1.087 118.3 116.3 103.6 114.5 0.023 2.2

rms (length) rms (angle) a

Bond lengths in

Y

A, angles in degrees.

TABLE III: Main Geometrical Features' of Minimal Conformer of VPA, VPDC, and VPO at MP2/6-31G* ~~

a

Bond lengths in

VPA

VPDC

VPO

1.340 1.780 1.489 1.086 1.085 1.087 119.2 117.9

1.340 1.785 1.484 1.086 1.086 1.087 118.3 116.3

1.340 1.800 1.501 1.086 1.086 1.089 119.2 115.7

A, angles in degrees.

TABLE IV: Mulliken Atomic Net Charges of the Minimal Conformers of VPA, VPDC, and VPO at the 6-31G* Level VPA

VPDC

VPO

-0.3446 -0.4300 1.4430 -0.7310 -0.7906

-0.3173 -0.4434 1.1112 -0.6370

-0.328 1 -0.4589 1.0322 -0.7329

-

0.2296 0.1988 0.2310 -0.1 152 1.3278

-

-0.2135 -

0.249 1 0.2175 0.2468 -0.0475 1.0637

-

-

-0.0775 0.2354 0.1987 0.2086 -0.144 0.8882

show a best agreement for STO-3G*. The relatively low agreement of the rms deviations for the bond angles found at 6-3 lG* and MP2/6-3 lG* levels with respect to the experimental data is owing to C-P=O and Cl-P==O bending angles. The main geometrical parameters of the three molecules are shown in Table 111. All of them are in the minimal conformation s-cis at the MP2/6-31G* level. The geometrical structures of VPA using 6-31G* and 6-31G** basis sets are very close. The substitutents of the phosphoryl group do not produce any significantdifference in the C=C bond length; however, the C-P and P=O bonds are longer in VPO, where the substituents of the phosphoryl group are less electronegative. The bond lengthvariationsas a function of the rotational motion around the C-P bond are, in general, very small, especially in the C=C double bond. This fact could indicate that the C=C/ P=O conjugative effect is not strong. Furthermore, the C-P bond shows a 0.007-A maximum change between the s-cis conformer and the next transition structure, strengthening the previous explanation. 4.3. ElectronicStructure. 4.3.1. Electronic Distribution. The Mulliken net charge distributions of the s-cis conformer of the VPA, VPDC, and VPO molecules at the 6-31G* level are shown in the Table IV. These charges reveal a highly polarized molecular structure. In all cases, a high positive net charge is observed on the phosphorus atom; therefore, all the atoms surrounding the

Figure 3. Molecular electrostatic potential (MEP) maps of the VPA molecule at the 6-31G** level.

phosphorus atom are strongly negatively charged. This effect is observed in similar molecules containing phosphorus This circumstance could be justified by the electronegativity difference among phosphorus and the other surrounding atoms. The positive net charge of the phosphorus follows the sequence VPA > VPDC > VPO. This fact is explained by the different electron-attractive effect of the substituents attached to the phosphorus atom. The net charge on the phosphoryl oxygen is generally higher than that on the hydroxylic oxygens. We remark that the atomic charge of the phosphoryl oxygens is lower than the other charges of the hydroxylic oxygens obtained at the 3-21G* and 6-31G* levels. These values could be considered an artifact of the Mulliken method. Hydroxylic oxygens show lower charges than the phosphoryl ones using the 6-3 lG* * basis set. Natural population (NPA) at the 6-31G* and 6-31G** levels, has also been performed and yields higher charges in the phosphoryl (-1.210) than in the hydroxylic oxygens (-1.082) at the 6-31G* level. In addition, MEP maps of VPA show that the phosphoryl oxygen is the most negative center of the system (Figure 3, the red color corresponds to the most negative values). The highest value of the MEP ranges from -52.7 to 82.1 kcal/mol with the 6-3 1G** basis set. The MEP has approximately the samevalues and distributionwith the 6-3 1G* basis set, showing the phosphoryl oxygen as the most negative atomic center of the molecule in all basis sets.

The Journal of Physical Chemistry, Vol. 98,No. 4, 1994 1113

Structure and Properties of Vinylphosphonic Acid

H

H

0

'?PA

VPDC

VP 0

Figure 4. Charge displacements and the dipole moments of the most stable conformers of VPA, VPDC, and VPO.

The C=C double bond is polarized toward the phosphorus atom, as can be seen from the net charge difference between both carbon atoms. The excess of negative net charge of the C1 with respect to the Cz comes from the phosphonic group. All the basis sets yield similar results. Compounds VPDC and VPO show a similar behavior. This polarity of the C=C double bond is in agreement with the @-orientedattack of nucleophilic reagents of vinylphosphonic group. l 6 The bond order47of the C-P bond in the s-cis conformer of VPA is slightly higher than that in the transition structure (0.964 and 0.958, respectively, at 6-3 1G*), and the C=C bond yields the same differences (1.944 and 1.937, respectively), which indicates that the conjugative effect in the C=C/P=O system is very weak. Molecules VPDC and VPO show the same behavior. In VPDC, the olefinic hydrogens have higher positive net charges, while the negative net charge of Cz is lower than that in the other molecules. In all the molecules and with all the studied basis sets, the Hg(cis) and HI gem) olefinic hydrogens have higher positive net charges than Hlo(trans) because of the higher proximity of the oxygen or chlorine atoms in the lowest conformer. However, in VPO, the Hll(gem) has a less positive charge because of the absent effect of the hydroxilic oxygen or chlorine atoms of the phosporylated group in VPA or VPDC, respectively. In Table I the dipole moments are shown, and in Figure 4 the charge displacements and the dipole moments of the most stable conformers of VPA, VPDC, and VPO are shown. In all cases and with all the basis sets, the dipole moment of the conformer s-cis is lower than the s-trans gauche secondary conformer. The -POXX (X = OH, C1, H ) group determines the dipole moments in these molecules. The different values of the dipole moments can be explained by taking into account the electronegativity of the X substituents in a simple geometrical model. Bonding in Phosphoryl Group. As a consequence of the hypervalence 5 of the phosphorus and the length of the P==O bond in the phosphonates, the phosphoryl bond is usually described as a double bond. The easiest way to explain the hypervalence of the phosphorus in phosphonates and other related compounds is to consider that the lone pair of the phosphorus atom in normal valence 3 is donated to an empty p orbital of the oxygen.48 In order to study this effect in the phosphoryl bond, the surface of electronic density of some canonical molecular orbital (CMO) at the 6-31G** level has been analyzed. Four CMOS have been found along the phosphoryl bond (two of u symmetry and two of P symmetry). Both of them should correspond to the phosphoryl bonds and the two lone pairs of the oxygen. If the phosphoryl bond were double, one of the orbitals of P symmetry and one of u symmetry should correspond to the single scheme of the -0. However, the remaining P bond, whichever of both is taken, presents their lobules along the P=O bond. At first sight, these pictures suggest not only a double bond but a triple bond. We have calculated, with the Boys procedure,35the localized molecular orbitals (LMO) at 6-31G**, 6-31G*, and STO-3G* basis sets. The centroids of these orbitals for the minimum conformer of the VPA are represented in Figure 5 . In all cases, the phosphoryl group shows the same peculiar electronic distribution: the LMOs show three centroids between the phosphorus and the phosphoryl oxygen atom, instead of two or

\

H

Figure 5. Centroids of LMO of the minimum conformer of VPA at the

6-31G** level.

one, as could be thought for a double or single bond, respectively. Furthermore, these centroids are located very close to the oxygen atom. These bonds would present a peculiar "banana" character, as Hillier et al. detected in the phosphine oxide.49 These results suggest a partial phosphoryl polarized triple bond. The L M O whose centroid is placed out of the phosphoryl bond is one of the lone pair of the oxygen. Gorden et al.50351 at the 3-21G* level and Molina et u I . a~t the ~ 6-31G* level found a similar bond in the phosphine oxide, which presents the same phosphoryl bond. The former authors also calculated L M O by the method of Edmiston-R~edenberg.~~ This last method yields a bond along the phosphoryl linkage and three lone pairs on the oxygen atom. They discussed both results and concluded that a polarized single bond is more appropriate for the phosphoryl linkage. In this compound, Kutzelnigg discussed the electronic structure of the phosphoryl bond, concluding that it is a partial triple bond.54~5sReed and S ~ h l e y e account r~~ for the hypervalence by means of a strong ionic component in the bond and a negative hyperconjugative effect (an electronic donation from the substituent orbitals to an antibonding molecular orbital, n u*). The highly polarized triple bond of the phosphoryl group can be explained56by means of a partially ionic u bond and P bonds through a negative hyperconjugative effect, n u* and n P*. The d orbitals on the phosphorus atom play only a secondary, but essential, role in the ?r character bonding.56 Boatz and Gordon5' found in the phosphine oxide that the phosphoryl bond is a single bond with a u donation from a phosphine lone pair strengthened by P back donation into the phosphorus. In the s ~ l f a m i d and e ~ ~1,2,6-thiadiazine- 1,l-dioxides,58the sulfoxide bond is single with a ?r back donation from the oxygen lone pairs to the sulfur atom. Taking into account the polarity of the PO bond (based on the net charges) and the electronegativity differences (P = 2.1 and 0 = 3 . 9 , this bond is in better agreement with a semipolar bond. Although we have seen that the Mulliken net charges do not sometimes yield reliable values, NPA corroborates this polarity. Streitwieser et al.59 also discuss this problem and consider that a dipolar structure as H3P+-O- could dominate over the -0 double bond. The calculated Mayer bond order47yields a value of 1.8 for VPA a t the 6-31G** basis set, in disagreement with a polarized single bond. It is well-known, when the bonds are polarized, that the Mayer bond order yields values lower than those for the nonpolar bonds. From the above arguments and in terms of MO, this bond should be described as a polarized triple bond, clearly different from the acetylene bond. The electronic structure for the phosphoryl bond (Pa+=O") appears whenever polarization functions are used (even at the STO-3G* level). This behavior is definitely not an artifact of the basis set,52,56,59 and it can be linked to the hypervalence phenomenon. The electronic structure of the phosphoryl group could be modified, in the vinylphosphonic acid, with respect to the

- -

Hernbndez-Laguna et al.

1114 The Journal of Physical Chemistry, Vol. 98, No. 4, 1994 TABLE V: Positions and Topological Characteristics of p(rc) of the R-PR' Bond of VPA at the 3-21G* molecule rc4 r((2-P). A1 A2 A3 t H~CHC-P(O)(OH)I 0.641 1.772 -0.269 -0.260 1.076 0.039 0.643 1.784 -0.2611 -0.2606 1.007 0.002 H3C-P(O)(OH)2 -0.626 -0.574 0.126 0.091 H2CHC-COOH H2CHC-CH3 -0.543 -0.529 0.089 0.028

and Some Related Molecular Systems Ihlh

P

V2P

0.248 0.259 4.968 6.101

0.171 0.170 0.295 0.274

0.546 0.485 -1.075 -0.983

GIP 1.497 1.435 0.254 0.220

With respect to the phosphorus atom in A. In atomic units.

TABLE VI: Position" and Topological Characteristics of p(rc) of the PO Bond of VPA and Some Related Molecular Systems at the 3-216* Levelb molecule (vinyl)(OH)#=O CH,(OH)2P=O H3P=Oc HP=O

(vinyl) (OH)C=O

(vinyl)(OH)(O)P-OH CH3(0H)(O)P-OH H2P-OH (vinyl)(O)C-OH 0

rcO

r(P0)"

A1

A2

A3

c

IAII/A3

P

0.590 0.590 0.592 (0.589) 0.595 0.621 0.621 0.632 -

1.452 1.452 1.459 (1.465) 1.469

-0.416 -0.416 -0.374 (-0.416) -0.385 -1.321 -0.310 -0.306 -0.225 -0.754

-0.409 -0.408 -0.374 (-0.416) -0.334 -1.176 -0.287 -0.285 -0.219 -0.706

3.227 3.230 3.088 (2.716) 3.115 3.102 1.990 1.983 1.616 1.442

0.016 0.019 0.000 (0.000) 0.153 0.123 0.081 0.076 0.028 0.068

0.129 0.129 0.121 (0.153) 0.123 0.426 0.156 0.154 0.139 0.523

0.225 0.225 0.216 (0.225) 0.205 0.434 0.166 0.165 0.148 0.305

-

1.584 1.585 1.630 -

V2P 2.402 2.405 2.340 (1.884) 2.396 0.605 1.393 1.392 1.172 -0.018

GlP 3.040 3.040 3.046 (2.636) 3.161 1.933 2.404 2.412 2.277 1.348

With respect to the phosphorus atom in A. In atomic units. The figures in brackets show the results obtained at the 6-31G8* basis set level.

phosphine oxide because of the existence of the A system of the vinyl group as well as the other oxygen atoms attached to it. Nevertheless, we have found three LMOs in the phosphoryl bond clearly shifted toward theoxygen. This phosphoryl bond structure could account for the weakness of the conjugative effect in the C=C/P=O system, because the overlap between the 3d and 3p orbitals of the phosphorus atom and the A orbital of the C=C bond, respectively, is small." In contrast, this overlap is high with the 2p orbitals of the phosphoryl oxygen atom.56 In the vinylphosphoryl dichloride and vinylphosphine oxide, we found the same phenomenon a t the 6-31G* and STO-3G* basis sets. 4.3.2. Topology of the Charge Density. In Tables V and VI, the topological features of p(rc)in the bond path of C-P and P-O bonds, respectively, are presented for VPA and related compounds at the 3-21G* level.60 VPA and methylphosphonic acid show V2p(rc)> 0, indicating a participation of ionic nature in the C-P bonds, although in less magnitude than in phosphoryl and P-OH bonds. Actually, all P-X bonds studied in this paper present V2p(rc) > 0, and the position of the critical points is closer to the phosphorus atom. The values of the ratio of kinetic energy density at the critical point and p(rc) are also included in Tables V and VI. These confirm the high polar nature of this bond because the relation G(rc)/p(rc)is higher than unity.20 We also observed values of the JA11/X3 ratio lower than unity, indicating a contraction of the charge toward the nuclei as corresponds with a participation of closed-shell interaction in these bonds. The C-P bond presents a non-null ellipticity in VPA ( f = 0.039), Table V, yielding a certain a character to this bond. This is compared with methylphosphonic acid, where it is close to zero, showing that the hyperconjugation between the methyl and phosphoryl groupsdoes not practically exist. The ellipticity value of the ( 4 ) - C in the propene molecule is accounted for by the allylic hyperconjugation. This last value is lower than that found in VPA, indicating that there is a certain A conjugation in the C=C/P=O system of VPA. The acrylic acid shows a f of 0,091 at the (=C)-C(=O) bond, as corresponds to a clearly ?r conjugated system. i is VPA is 43% that of acrylic acid. Therefore, we conclude that a weak A conjugation exists in the C=C/P=O system of VPA, in agreement with Kabachnik.16 Bondingin Phosphoryl Group. The VZp(rc)values are higher than unity in all the PO bonds in VPA and related compounds, Table VI. Furthermore, thephosphoryl bonds present the highest values, showing that they are more polar than P-OH single bonds. Wiberg et al. found the same in the carbonyl bonds, and concluded

that the carbonyl linkage could be best represented by a dipolar structure.6l However, the value in acrylic acid is not higher than that in the phosphoryl. The same happens with the G(rc)/p(rc) ratio. The IAl(/X3 quotients are smaller than unity, with those of the phosphoryl bonds being the smallest. All of these magnitudes indicate the highly polar nature of the phosphoryl bond. In Table VI, the ellipticities of the P-OH single bonds and phosphoryl bonds in trivalent and pentavalent phosphorus are matched. In trivalent phosphorus compounds, the P-OH single bond in the phosphinous acid (HzPOH) shows a small value of f ; however the values of the P-OH bonds are higher in pentavalent phosphorus compounds. In acrylic acid, the f of the C-OH bond presents a value similar to that of the pentavalent phosphorus P-OH bonds. In a clearly PO double bond, in a trivalent phosphorus compound, as in the oxophosphine (HPO), [ reaches a value of 0.153, which corresponds to a typical double bond. In the carbonyl bond of the acrylic acid, f is similar. The negative curvatures decrease in the same way as those of ethane and eth~1ene.l~ However, the positive curvature increases in a manner opposite than that of ethane and ethylene. This fact could be explained by the polarity of the PO bond and because the charges are more separated, and because A3 increases. The f of the phosphoryl bond in the pentavalent phosphorus compounds, such as phosphine oxide, VPA, and methylphosphonic acid, has very low values, being zero in phosphine oxide (as in acetylene). This shows that the second ?r orbital is equivalent to the first, and so the phosphoryl bond reaches an axial symmetry.I9 In addition X1 and A2 of this bond are, in general, lower than those of P-OH bonds and the P==Obond in the oxophosphine molecule. The same behavior is observed in acetylene with respect to ethane and ethylene, although the differences in the curvatures between these molecules are higher. However, the X3's of phosphoryl bond are higher than those of the single and double PO bonds, because of the greater polarization of the phosphoryl bond. All the = 0.028 in ellipticities of the P-OH single bonds are nonzero (I H2P-OH). From these results and the unusual short bond length (according to the experimental for the phosphoryl bond (1.45 A) versus the P-O(H) bond length (1.58 A), the phosphoryl bond in pentavalent phosphorus compounds behaves as a triple bond. Taking into account the high polarity shown by this bond, it might be assigned a partial polarized triple-bond character (R(X)2P6+=Ob (R = -C=C-; X = OH, C1, H)), in agreement with Kutzelnigg et a1.,54,55 who found the same in the phosphine oxide.

Structure and Properties of Vinylphosphonic Acid

The Journal of Physical Chemistry, Vol. 98, No. 4, 1994 1115

TABLE VII: Ultraviolet Absorptions of VPA and Analogous Compounds "pd

H2CNHCOOR[64] H2C4HPOpHp H2C4H-CH2POpH2

solv. ethanol methanol methanol

(nm) 195 204 203

e

13804 179 30

4.4. Comparison with Experimental Data. In Table VII, UV spectroscopic absorption data of vinylphosphonic acid and analogous compounds are shown. The molar absorption coefficient (e) of vinylphosphonic acid (VPA) shows a very small value. This is much lower than that in the acrylic derivative^.^^ It is well-known that conjugative systems elevate UV absorption. In contrast, the VPA absorption is higher than that of the allylphosphonic acid, where there is no possibility of this kind of conjugative effect. These facts mean that the ?r conjugation between C = C and P=O exists but is weak. This result is in agreement with the ab initio theoretical outcomes, where the conformational analysis, the geometry, electronic structure, and topological analysis show also a weak C=C/P=O conjugation. The 13C-NMR data of vinylphosphonic acid show that the C=C double bond is very polarized toward the phosphorus. The C1(6 = 136.7 ppm) is less shielded than the C1 (6 = 129.6 ppm), in agreement with the theoretical results. In the VPDC, the ab initio theoretical data agree fairly well with the experimental results, from gas-phase electronic diffraction,13 where the more stable conformer is found to be the s-cis one.

5. Conclusions The ab initio calculations (conformational analysis, geometry, electronic structure, and topological analysis) and the experimental results presented in this paper are in agreement for the vinylphosphonic derivatives (VPA, VPDC, and VPO). The three compounds exhibit the planar s-cis configuration as the most stable conformer and a nonplanar s-trans-gauche conformation as the second conformer with a low energy difference. The two rotational barriers in each compound are also low. The vinylphosphonic derivatives appear to be strongly polarized molecules. In particular, the phosphoryl bond in VPA, VPDC, VPO, and several related compounds is polarized and should be represented by a partially polarized triple bond. The C=C/ P=O system presents a very weak conjugation.

Acknowledgment. We are grateful to Profs. J. Molina-Molina and J. A. Dobado of the University of Granada for computational facilities and fruitful discussions and to Prof. M. S.Arias-Perez of the University of Alcali5de Henares for the valuable information on N M R data. This work was supported by Grant PB90-0279 from Spanish DGICYT. References and Notes (1) Christensen, B. G.; Leanza, W. J.; Beattle, T. R.; Patchott, A. A,; Arison, B. H.; Ormard, E.; Kuehl, F. A.; Aldersschonberg, G.; Jardetzky, 0. Science 1969, 166, 123. (2) Smeyers, Y. G.; Hernbndez-Laguna, A.; Von Carstenn-Lichterfelde, C. J . Pharm. Sci. 1983, 72, 1011. (3) Smeyers, Y. G.; Hernbndez-Laguna, A.; Romero-Sbnchez, F. J.; FernBndez-IbBfidez,M.; Gblvez-Ruano, E.; Arias-PCrez, M. S . J.Pharm. Sci. 1987, 76, 753. (4) Hemmi, K.; Takeno, H.; Hashimoro, M.; Kamiya, T. Chem. Pharm. Bull. 1982, 30, 111. ( 5 ) Eto, M. Organophosphorus pesticides. Organic and Biological Chemistry; CRC: Boca Raton, FL, 1974; pp 329-67. (6) Zyablikova, T. A,; Il'yasov, A. V.; Mukhametzyanova, E. Kh.; Shersnergorn, I. M. J . Gen. Chem. USSR 1981, 249. (7) Welch, C. M.; Gonzalez, E. J.; Guthrie, J. D. J . Org. Chem. 1961, 26, 3270. (8) Degenhart, C. R.; Burdsall, D. C. J. Org. Chem. 1986, 51, 3488.

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(53) Edmiston, C.; Ruedenberg, K. Rev. Mod. Phys. 1963, 35, 457. Edmiston, C.; Ruedenberg, K. J. Chem. Phys. 1965,43, 597. (54) (a) Kutzelnigg, W. Pure Appl. Chem. 1977,49,981. (b) Wallmeier, H.:Kutzelniaa. W. J. Am. Chem. SOC.1979, 101, 2804. '(55) Kut&igg, W. Angew. Chem., Znr. Ed. Engl. 1984, 23, 272. (56) Reed, A. E.; Schleyer, P. v. R. J. Am. Chem. SOC.1990,112, 1434. (57) M6, 0.;de Paz, J. L. G.; Yifiez, M.; Alkorta, I.; Elguero, J.; Goya, P.; Rozas, I. Can. J. Chem. 1989, 67, 2227. (58) Elguero, J.; Goya, P.;Martfnez, A,; Rozas, I.; M6, 0.;de Paz, J. L. G.; Yaiiez, M. J. Phys. Org. Chem. 1990,3,470. (59) Streitwieser, A.; Rajca, A.; McDowell, R. S.;Glaser,R. J. Am. Chem. Soc. 1987, 109, 4184.

Hernindez-Laguna et al. (60) In order to compare the results, we use the same basis set as Gordon et aLS0 Nevertheless, in general, the trends in the behavior of topological featuresofthechargedensityarepreservtdbetweenbasissets.~~ Thephosphine oxide has been calculated at 3-21G' and 6-31G** levels, Table VI. (61) Wiberg, K.B.; Breneman, C. M.; LePage, T. J. J. Am. Chem. SOC. 1990,112, 61, (62) DeLaMatter, D.; McCullough, J. J.; Calvo, C. J. Phys. Chem. 1973, 77, 1146. (63) Weakley, T. J. R. Acra Crystallogr. ( B ) 1976, 32, 2889. (64) Brunn, J.; Dethloff, M.; Riebenstahl, H. Z. Phys. Chem. ( h i p i g ) 1977, 258, 209.