molecules Article UnusualArticle Complexes of P(CH)3 with FH, ClH, and ClF 3 JanetUnusual E. Del Bene Complexes1,* , Ibon Alkorta 2,of* andP(CH) José Elguero with2 FH, ClH, and ClF Janet1 Department E. Del Bene of Chemistry,1,*, Ibon Alkorta Youngstown 2,* and State José University, ElgueroYoungstown, 2 OH 44555, USA 2 Instituto de Química Médica (IQM-CSIC), Juan de la Cierva, 3, E-28006 Madrid, Spain; [email protected] 1 Department of Chemistry, Youngstown State University, Youngstown, OH 44555, USA * Correspondence: [email protected] (J.E.D.B.); [email protected] (I.A.); Tel.: +330-609-5593 (J.E.D.B.); 2 Instituto de Química Médica (IQM-CSIC), Juan de la Cierva, 3, E-28006 Madrid, Spain; [email protected] +34-91562-2900 (I.A.) * Correspondence: [email protected] (J.E.D.B.); [email protected] (I.A.); Tel.: +330-609-5593 (J.E.D.B.); +34-91562-2900 (I.A.) Received: 27 May 2020; Accepted: 16 June 2020; Published: 19 June 2020 Received: 27 May 2020; Accepted: 16 June 2020; Published: 19 June 2020 Abstract: Ab initio MP2/aug’-cc-pVTZ calculations have been performed to determine the structures andAbstract: binding Ab energiesinitio MP2/aug’-cc-pVTZ of complexes formed calculations by phosphatetrahedrane, have been performed P(CH)to determine3, and HF,the HCl,structures and ClF.and Fourbinding types energies of complexes of complexes exist onformed the potential by phosphatetrahedrane, energy surfaces. P(CH) Isomers3, andA form HF, HCl, at the and P atom ClF. nearFour thetypes end of complexes of a P-C bond, exist onB atthe a potential C-C bond, energyC at surfaces. the centroid Isomers of theA form C-C-C at the ring P atom along near the the C3 symmetryend of a P-C axis, bond, and B Dat ata C-C the Pbond, atom Calong at the thecentroid C3 symmetry of the C-C-C axis. ring Complexes along theA C3and symmetryB are stabilized axis, and byD at hydrogen the P atom bonds along when the C FH3 symmetry and ClH areaxis. the Complexes acids, and A byand halogen B are stabilized bonds when by hydrogen ClF is the bonds acid. Inwhen isomers FH andC, theClH dipole are the moments acids, and of theby twohalogen monomers bonds when are favorably ClF is the aligned acid. In but isomers in D the C, alignmentthe dipole ismoments unfavorable. of the two For monomers each of the are monomers, favorably aligned the binding but in energies D the alignment of the complexes is unfavorable. decrease For each in the of orderthe monomers,A > B > C the> Dbinding. The most energies stabilizing of the Symmetrycomplexes Adapteddecrease in Perturbation the order A Theory > B > C (SAPT) > D. The binding most energystabilizing component Symmetry for Adapted the A Perturbationand B isomers Theory is the (SAPT) electrostatic binding interaction,energy component while thefor the dispersion A and B interactionisomers is the is theelectrostatic most stabilizing interaction, term while for C theand dispersionD. The barriers interaction to converting is the most one stabilizing isomer toterm another for C areand significantly D. The barriers higher to forconverting the A isomers one isomer compared to another to B. Equation are significantly of motion higher coupled for clusterthe A isomers singles andcompared doubles to B (EOM-CCSD). Equation of motion intermolecular coupled couplingcluster singles constants and doubles J(X-C) (EOM-CCSD) are small for bothintermolecularB and C isomers.coupling J(X-P)constants values J(X-C) are are larger small and for positive both B inand the C Aisomers.isomers, J(X-P) negative values in are the largerB isomers, and positive and have in theirthe A largest isomers, positive negative values in the inthe B isomers,D isomers. and Intramolecular have their largest coupling positive constants values1 J(P-C)in the experienceD isomers. 1 littleIntramolecular change upon coupling complex constants formation, J(P-C) except experience in the halogen-bondedlittle change upon complex complex FCl:P(CH formation,3) Aexcept. in the halogen-bonded complex FCl:P(CH3) A. Keywords: hydrogen bonds; halogen bonds; dipole interactions; coupling constants Keywords: hydrogen bonds; halogen bonds; dipole interactions; coupling constants 1.1. Introduction SymmetrySymmetry in Chemistry is is one one of of the the cornerstones cornerstones of of chemistry, chemistry, both both in in terms terms of of the the beauty beauty it itbestows bestows on on molecules, molecules, and and its its importance importance for for life. life. There There is is no no question that highlyhighly symmetricalsymmetrical moleculesmolecules are intrinsicallyintrinsically beautiful and fascinating.fascinating. Moreover, symmetry in chemistry, particularlyparticularly chirality,chirality, isis importantimportant forfor lifelife itself,itself, asas firstfirst demonstrateddemonstrated byby PasteurPasteur [[1–4].1–4]. In additionaddition to thethe highlyhighly symmetricalsymmetrical ArchimedeanArchimedeanstructures structures mainly mainly known known thanks thanks to the to fullerenes, the fullerenes, the five Platonicthe fivestructures Platonic whichstructures do notwhich have do atoms not have in the atoms center, in the but center, only on but the only periphery, on the periphery, are also intrinsically are also intrinsically beautiful. Thebeautiful. Platonic The structures Platonic arestructures illustrated are illustrated in Figure1. in Figure 1. Figure 1. Five Platonic structures: tetrahedron, octahedron, hexahedron (cube), dodecahedron, andFigure icosahedron. 1. Five Platonic structures: tetrahedron, octahedron, hexahedron (cube), dodecahedron, and icosahedron. Octahedron-type molecules are unknown, [1] but icosahedron geometries are found in boron Moleculesderivatives2020 , 25such, 2846; as doi: the10.3390 dodecaborate/molecules25122846 dianion [2] and the closo-carboraneswww.mdpi.com [3]. The/journal three/molecules other Molecules 2020, 25, 2846; doi:10.3390/molecules25122846 www.mdpi.com/journal/molecules Molecules 2020, 25, 2846 2 of 14 Octahedron-type molecules are unknown, [1] but icosahedron geometries are found in boron derivatives such as the dodecaborate dianion [2] and the closo-carboranes [3]. The three other structures could exist as pure carbon derivatives, although in some cases, with external substituents needed for stabilization. These include tetrahedrane (CH)4,[4,5] cubane (CH)8,[6–8] and dodecahedrane (CH)20 [9–11]. Chemists have searched the Periodic Table for structures which could be obtained by replacing C or CR by X or XR. Theoretical studies of tetrahedrane molecules in which C is replaced by elements from Si to Pb and from B to Tl, [12,13] CR is replaced by SiR or GeR, [9] or CH is replaced by N, have demonstrated that these are stable systems [14–16]. An early study of replacing CR by P was carried out on phosphacubanes by Reitz et al. [17]. Di-tert-butyldiphosphatetrahedrane has been recently synthesized and characterized by Wolf et al. [18]. White phosphorus, P4, which has a tetrahedral structure, has been the subject of many theoretical studies [19–22]. As is the case for C4, all tetrahedral (CR)4 molecules lack a dipole moment. This usually leads to weaker noncovalent interactions, even in cases in which the interacting molecule may have a large positive σ-hole. Yáñez demonstrated that phosphatetrahedrane and diphosphatetrahedrane behave as carbon bases in the gas phase [23]. Boldyrev et al. studied the molecules CnHnP4–n (n = 0–4) and their transformation into planar structures [24]. We have investigated the P4 molecule in complexes stabilized by hydrogen, halogen, and pnicogen bonds [25]. The “elusive phosphatetrahedrane” was finally isolated by Cummins et al. [26] in the form of the tris-tert-butyl derivative. In the present study, we have searched the potential energy surfaces of phosphatetrahedrane, P(CH)3 interacting with FH, ClH, and ClF. The structures and binding energies of the isomers that exist on these surfaces have been determined, as well as the transition structures that convert one equilibrium isomer to another. To gain further insight into the binding energies of these complexes, SAPT analyses have been carried out. Finally, EOM-CCSD spin–spin coupling constants across intermolecular bonds have been computed for all complexes. It is the purpose of this paper to present the results of this study. 2. Results and Discussion 2.1. Parent Molecule P(CH)3 and Its Molecular Electrostatic Potential (MEP) The parent molecule P(CH)3 is pyramidal with C3v symmetry. Although the experimental structure of this molecule is not known, a derivative P(C-t-butyl)3 has been synthesized. An X-ray diffraction study of this molecule reported P-C distances of 1.837 to 1.860 Å, and C-C bond distances of 1.467 to 1.481 Å. The computed P-C distances of P(CH)3 are 1.857 Å and the C-C distances are 1.465 Å. In addition, the one-bond coupling constant 1J(P-C) has been measured experimentally and 1 found to have an unsigned value of 37.9 Hz. The computed EOM-CCSD value of J(P-C) for P(CH)3 is –40.3 Hz. Thus, the computed structure and P-C coupling constant for P(CH)3 are consistent with the experimental values for the t-butyl derivative. The MEP of the P(CH)3 molecule is illustrated in Figure2. A triply degenerate MEP minimum with a value of 0.018 au is located at the P atom almost perpendicular to the C symmetry axis. − 3 Three additional triply degenerate minima with values of 0.006 au are associated with the midpoint − of each C-C bond in the ring. There is also a minimum of 0.005 au located on the C axis at the P − 3 atom. At these minima, P(CH)3 should act as an electron donor. There are also two unique maxima on this surface. One is a three-fold degenerate MEP maximum associated with the three C-H bonds.