Heats of formation of [2.2]paracyclophane-1-ene and [2.2]paracyclophane-1,9-diene - an experimental study

The enthalpies of formation [Delta(g)] of tricyclo[8.2.2.2(4,7)]hexadeca-1(13),2,4(16),5,7(15),10(14),11-heptaene (2, 1,2-dehydro[2.2]paracyclophane or [2.2]paracyclophane-1-ene) and tricyclo[8.2.2.2(4,7)]hexadeca-1(13),2,4(16),5,7(15),8,10(14),11-octaene (3, 1,2,9,10-dehydro[2.2]paracyclophane or [2.2]paracyclophane-1,9-diene) have been determined by measuring their heats of combustion in a microcalorimeter and their heats of sublimation by the transpiration method. Values of the strain energies (SE) [SE(2) = 34.7 kcal mol(-)(1), SE(3) = 42.0 kcal mol(-)(1)] have been derived from the gas-phase heats of formation and are compared with those from MM3 and PM3 calculations and with the corresponding value SE(1) = 30.1 kcal mol(-)(1) for the parent tricyclo[8.2.2.2(4,7)]hexadeca-1(13),4(16),5,7(15),10(14),11-hexaene (1, [2.2]paracyclophane). The higher strain energies of 2 and 3 (by 4.6 and 11.9 kcal mol(-)(1)) are in accord with the well-known increased reactivities of their aromatic rings as a consequence of their increased bending. As revealed by an X-ray crystal structure analysis, the bending in the monoene 2 corresponds to that of 1 and 3 at one of two bridging corners.     

Generation and reactions of two new functionalized tricyclo[3.3.0.0 3,7]oct-1(5)-ene derivatives

The generation of the new functionalized and highly pyramidalized alkenes, 3,7-(2,2'-biphenylene)tricyclo[3.3.0.0(3,7)]oct-1(5)-ene (20) and 3,7-sulfonyldioxytricyclo[3.3.0.0(3,7)]oct-1(5)-ene (39), and their trapping with 1,3-diphenylisobenzofuran and 11,12-dimethylene-9,10-dihydro-9,10-ethanoanthracene are described. While both alkenes 20 and 39 failed to give the expected cyclobutane or diene dimers, 20 was reacted with 3,7-dimethyltricyclo[3.3.0.0(3,7)]oct-1(5)-ene (1b) to give the cross-coupled product 4,5-(2,2')-biphenylene-10,11-dimethylpentacyclo[8.2.1.1(2,5).1(4,7).1(8,11)]hexadeca-1,7-diene (33). DFT calculations [B3LYP/6-31G(d)] on compound 20 gave important parameters of this pyramidalized alkene, such as the pyramidalization angle (61.7 degrees), the strain energy (72.9 kcal/mol), and the HOMO/LUMO gap (3.79 eV).     

Reactions of sterically congested 1,5-hexadienes: Ab initio and DFT calculations on the competition between cope rearrangements and disrotatory cyclobutene ring-opening reactions of bridged syn-tricyclo[4.2.0.0(2,5)]octa-3,7-dienes

The thermolytic behavior of four syn-tricyclo[4.2.0.0(2,5)]octa-3,7-dienes, each spanned by four propano bridges (13, 14, 21, and 26), has been investigated by means of calculations at the UB3LYP/ 6-31G* and CASPT2/6-31G levels. These calculations predict that 13 should undergo a degenerate Cope rearrangement (E(A) = 19.6 kcal/mol), whereas the other three C(20)H(24) isomers should prefer a necessarily disrotatory cyclobutene ring-opening reaction. In the case of 14, the ring-opening reaction (E(A) = 27.2 kcal/mol) is concerted and leads directly to 15, a 4-fold bridged cyclooctatetraene. In the ring opening of 21, the 1,6-bridge in the 4-fold bridged bicyclo[4.2.0]octa-2,4,7-triene 31 prevents formation of the corresponding cyclooctatetraene. In the ring opening of 26, the 4-fold bridged bicyclo[4.2.0]octa-2,4,7-triene derivative 36 is predicted to form the corresponding bridged cyclooctatetraene 38, which should undergo bond shift. The results of these calculations are found to be in very good agreement with the results of experiments on these hydrocarbons.     

Pseudo-bimolecular [2+2] cycloaddition studied by time-resolved photoelectron spectroscopy

The first study of pseudo‐bimolecular cycloaddition reaction dynamics in the gas phase is presented. We used femtosecond time‐resolved photoelectron spectroscopy (TRPES) to study the [2+2] photocycloaddition in the model system pseudo‐gem‐divinyl[2.2]paracyclophane. From X‐ray crystal diffraction measurements we found that the ground‐state molecule can exist in two conformers; a reactive one in which the vinyl groups are immediately situated for [2+2] cycloaddition and a nonreactive conformer in which they point in opposite directions. From the measured S₁ lifetimes we assigned a clear relation between the conformation and the excited‐state reactivity; the reactive conformer has a lifetime of 13 ps, populating the ground state through a conical intersection leading to [2+2] cycloaddition, whereas the nonreactive conformer has a lifetime of 400 ps. Ab initio calculations were performed to locate the relevant conical intersection (CI) and calculate an excited‐state [2+2] cycloaddition reaction path. The interpretation of the results is supported by experimental results on the similar but nonreactive pseudo‐para‐divinyl[2.2]paracyclophane, which has a lifetime of more than 500 ps in the S₁ state.     

Transition states for the dimerization of 1,3-cyclohexadiene: a DFT, CASPT2, and CBS-QB3 quantum mechanical investigation

Quantum mechanical calculations using restricted and unrestricted B3LYP density functional theory, CASPT2, and CBS-QB3 methods for the dimerization of 1,3-cyclohexadiene (1) reveal several highly competitive concerted and stepwise reaction pathways leading to [4 + 2] and [2 + 2] cycloadducts, as well as a novel [6 + 4] ene product. The transition state for endo-[4 + 2] cycloaddition (endo-2TS, DeltaH(double dagger)(B3LYP(0K)) = 28.7 kcal/mol and DeltaH(double dagger)(CBS-QB3(0K)) = 19.0 kcal/mol) is not bis-pericyclic, leading to nondegenerate primary and secondary orbital interactions. However, the C(s) symmetric second-order saddle point on the B3LYP energy surface is only 0.3 kcal/mol above endo-2TS. The activation enthalpy for the concerted exo-[4 + 2] cycloaddition (exo-2TS, DeltaH(double dagger)(B3LYP(0K)) = 30.1 kcal/mol and DeltaH(double dagger)(CBS-QB3(0K)) = 21.1 kcal/mol) is 1.4 kcal/mol higher than that of the endo transition state. Stepwise pathways involving diallyl radicals are formed via two different C-C forming transition states (rac-5TS and meso-5TS) and are predicted to be competitive with the concerted cycloaddition. Transition states were located for cyclization from intermediate rac-5 leading to the endo-[4 + 2] (endo-2) and exo-[2 + 2] (anti-3) cycloadducts. Only the endo-[2 + 2] (syn-3) transition state was located for cyclization of intermediate meso-5. The novel [6 + 4] "concerted" ene transition state (threo-4TS, DeltaH(double dagger)(UB3LYP(0K)) = 28.3 kcal/mol) is found to be unstable with respect to an unrestricted calculation. This diradicaloid transition state closely resembles the cyclohexadiallyl radical rather than the linked cyclohexadienyl radical. Several [3,3] sigmatropic rearrangement transition states were also located and have activation enthalpies between 27 and 31 kcal/mol.     

A bridged triptycene-intramolecular diels-alder reaction of 16,19-dimethoxy-[2](9,10)anthraceno[2]paracyclophane

A bridged triptycene was obtained by intramolecular cycloaddition reaction of 16,19-dimethoxy[2](9,10)anthraceno[2]paracyclophane followed by acid-catalyzed elimination of methanol. Internal strain in this molecule was disclosed by X-ray crystallographic analysis.     

Reaction mechanism and chemoselectivity of intermolecular cycloaddition reactions between phenyl-substituted cyclopropenone ketal and methyl vinyl ketone

In this paper, the mechanisms of the intermolecular [3+2] and [1+2] cycloaddition reactions of 1,1/1,3-dipolar π-delocalized singlet vinylcarbenes, which is obtained from cyclopropenone, with an electron-deficient C═O or C═C dipolarophile, to generate five-membered ring products are first disclosed by the density functional theory (DFT). Four reaction pathways, including two concerted [3+2] cycloaddition reaction pathways and two stepwise reaction pathways (an initial [1+2] cycloaddition and then a rearrangement from the [1+2] cycloadducts to the final [3+2] cycloadducts), are investigated at the B3LYP/6-31G(d,p) level of theory. The calculated results reveal that, in contrast to the concerted C═O [3+2] cycloaddition reaction pathway, which is 7.1 kcal/mol more energetically preferred compared with its stepwise reaction pathway, the C═C dipolarophile favors undergoing [1+2] cycloaddition rather than concerted [3+2] cycloaddition (difference of 5.3 kcal/mol). The lowest free energy barrier of the C═O concerted [3+2] cycloaddition reaction pathway shows that it predominates all other reaction pathways. This observation is consistent with the finding that the C═O [3 + 2] cycloadduct is the main product under experimental conditions. In addition, natural bond orbital second-order perturbation charge analyses are carried out to explain the preferred chemoselectivity of C═O to the C═C dipolarophile and the origins of cis-stereoselectivity for C═C [1+2] cycloaddition. Solvent effects are further considered at the B3LYP/6-31G(d,p) level in the solvents CH(3)CN, DMF, THF, CH(2)Cl(2), toluene, and benzene using the PCM model. The results indicate that the relative reaction trends and the main products are insensitive to the polarity of the reaction solvent.     

DFT (density functional theory) studies on cycloisomerization of 15–membered triazatriacetylenic macrocycle

The mechanism as well the stereochemistry of cascade cycloisomerization of 15–membered triazatriacetylenic macrocycle was investigated theoretically by using M062X/6–31+G(d,p) and M062X/LANL2DZ calculations. The results showed that the mechanism and outcome of the reaction depended on the absence and presence of a transition metal catalyst. So that, in thermal-induced condition, the reaction had to experience several suprafacial concerted reactions including Ene-reaction (DG^#=35.38 kcal/mol), Diels–Alder cycloaddition (DG^# = 17.16 kcal/mol), and sigmatropic H-shift rearrangement (DG^# = 56.21 kcal/mol) to produce diastereoselective fused cis–tetracyclic aromatic bearing a pyrrole moiety by following kinetic considerations. Also, the [2+2+2] cycloaddition mechanism was neglected in thermal–induced conditions because of high activation free Gibbs energy (DG^# = 63.90 kcal/mol). In the presence of palladium catalyst, Pd(0) formed an adduct by coordinating to C = C bonds and decreased the DG^# of the process to 29.58 kcal/mol, and consequently provided a facilitated media for the reaction to follow the [2+2+2] to produce more stable fused tetracyclic benzenoid aromatic by passing through the lower energy barrier.     

MP2 and DFT calculations on circulenes and an attempt to prepare the second lowest benzolog, [4]circulene

MP2 and DFT calculations have been carried out for [n]circulenes for n=3 to 20 in order to predict the strain energy and topology of these cyclically condensed aromatic systems. To synthesise [4]circulene (2), 1,5,7,8-tetrakis(bromomethyl)biphenylene (14) was prepared from the corresponding tetramethyl derivative (8) and subjected to various dehalogenation reactions; all attempts to obtain [2.2]biphenylenophane (7) as a precursor for 2 by this route failed. Treatment of 14 with sodium sulfide furnished the thiaphanes 16 and 17, thermal and photochemical desulfurization of which also failed to provide 7. In a second approach [2.2]paracyclophane was converted to the pseudo-geminal dithiol 23, which was subsequently bridged to the thiaphanes 22 and 24. On flash vacuum pyrolysis at 800 degrees C these were converted exclusively into phenanthrene (30). An approach to dehydrochlorinate the commercial product PARYLENE C to the tetrahydro[4]circulene 7 led only to polymerisation. The X-ray structures of the intermediates 8, 14, 17, 23, 24, 26, and 35 are reported.     

Chemo- and periselectivity in the addition of [OsO2(CH2)2] to ethylene: a theoretical study

Quantum chemical calculations by using density functional theory at the B3LYP level have been carried out to elucidate the reaction course for the addition of ethylene to [OsO2(CH2)2] (1). The calculations predict that the kinetically most favorable reaction proceeds with an activation barrier of 8.1 kcal mol(-1) via [3+2] addition across the O=Os=CH2 moiety. This reaction is -42.4 kcal mol(-1) exothermic. Alternatively, the [3+2] addition to the H2C=Os=CH2 fragment of 1 leads to the most stable addition product 4 (-72.7 kcal mol(-1)), yet this process has a higher activation barrier (13.0 kcal mol(-1)). The [3+2] addition to the O=Os=O fragment yielding 2 is kinetically (27.5 kcal mol(-1)) and thermodynamically (-7.0 kcal mol(-1)) the least favorable [3+2] reaction. The formal [2+2] addition to the Os=O and Os=CH2 double bonds proceeds by initial rearrangement of 1 to the metallaoxirane 1 a. The rearrangement 1-->1 a and the following [2+2] additions have significantly higher activation barriers (>30 kcal mol(-1)) than the [3+2] reactions. Another isomer of 1 is the dioxoosmacyclopropane 1 b, which is 56.2 kcal mol(-1) lower in energy than 1. The activation barrier for the 1-->1 b isomerization is 15.7 kcal mol(-1). The calculations predict that there are no energetically favorable addition reactions of ethylene with 1 b. The isomeric form 1 c containing a peroxo group is too high in energy to be relevant for the reaction course. The accuracy of the B3LYP results is corroborated by high level post-HF CCSD(T) calculations for a subset of species.     

Mechanistic Study on the Monofunctional Binding of Hydrolysis Products of Non-planar Heterocyclic Complexes Trans-[PtCl_2NH_3（piperidine）] and Trans-[PtCl_2（piperidine）_2] to DNA Purine Bases

The monofunctional substitution reactions between trans-[PtCl(H2O)(NH3)(pip)]+,trans-[Pt(H2O)2(NH3)(pip)]2+,trans-[PtCl(H2O)(pip)2]+,trans-[Pt(H2O)2(pip)2]2+ (pip = piperidine) and adenine/guanine nucleotides are explored by using B3LYP hybrid functional and IEF-PCM salvation models. For the trans-[Pt(H2O)2(NH3)(pip)]2+ and trans-[PtCl(H2O)(NH3)(pip)]+ complexes,the computed barrier heights in aqueous solution are 13.5/13.5 and 11.6/11.6 kcal/mol from trans-Pt-chloroaqua complex to trans/cis-monoadduct for adenine and guanine,and the corresponding values are 20.7/20.7 and 18.8/18.8 kcal/mol from trans-Pt-diaqua complex to trans/cis-monoadduct for adenine and guanine,respectively. For trans-[PtCl(H2O)(pip)2]+ and trans-[Pt(H2O)2(pip)2]2+,the corresponding values are 21.5/21.3 and 19.4/19.4 kcal/mol,and 26.0/26.0 and 20.7/20.8 kal/mol for adenine and guanine,respectively. Our calculations demonstrate that the barrier heights of chloroaqua are lower than the corresponding values of diaqua for adenine and guanine. In addition,the free energies of activation for guanine in aqueous solution are all smaller than that for adenine,which predicts a preference of 1.9 kcal/mol when trans-[PtCl(H2O)(NH3)(pip)]+ and trans-[Pt(H2O)2(NH3)(pip)]2+ are the active agents and ～1.9 and ～ 5.3 kcal/mol when trans-[PtCl(H2O)(pip)2]+ and trans-[Pt(H2O)2(pip)2]2+ are the active agents,respectively. For the reaction of trans-Pt-chloroaqua (or diaqua) to cis-monoadduct,we obtain the same transition-state structure as from the reaction of trans-Pt-chloroaqua (or diaqua) to trans-monoadduct,which seems that the trans-Pt-chloroaqua (or diaqua) complex can generate trans-or cis-monoadduct via the same transition-state.     

Photochemical synthesis of [2.2](3,8)-pyridazinophane and quinolinophane-2(1H)-one as well as synthesis of [2](5,8)-quinolinophanes and fused spiro-pyranoindanoparacyclophanes

Syntheses of various classes of unreported heterophanes derived from [2.2]paracyclophane are herein reported. The key to their successful synthesis depends on the photochemical synthesis of pyridazinophane and quinolinophane-2(1H)-one from freshly prepared 4-([2.2]paracyclophanyl)-azo-4′-[2.2]paracyclophane and 4-([2.2]paracyclophanyl)cinnnamanilide, respectively. Reactions of 4-amino-[2.2]paracyclophane with either acetyl- or benzoylacetone afforded condensed products. Then ring closure using polyphosphoric acid (PPA) at 120°C gave, in near quantitative yields, quinolinophanes. Reactions of [2](4,7)-indano-[2]paracyclophane-1-ylidene-propanedinitrile with active methylene compounds afforded fused spiro-pyranoindanoparacyclophane derivatives.     

Structure Determination of an Unusual [2.2]paracyclophane Derivative by NMR Spectroscopy

Our ongoing study on cycloaddition reactions of dienes with different dienophiles afforded a great variety of derivatives with interesting molecular structures and electronic behavior. A new type of angularly annelated [2.2]paracyclophane (3) has been synthesize by the Diels–Alder reaction of 4-(2-propenyl[2.2]paracyclophane (1) and 1,4-benzoquinone (2) under high pressure conditions. The structure determination of this compound has been achieved by NMR measurements and semiempirical calculations.     

Synthesis, characterization, and spectroscopy of 4,7,12,15-[2.2]paracyclophane containing donor and acceptor groups: impact of substitution patterns on through-space charge transfer

This paper reports the synthesis of 4,7,12,15-tetra(4'-dihexylaminostyryl)[2.2]paracyclophane (1), 4-(4'-dihexylaminostyryl)-7,12,15-tri(4' '-nitrostyryl)[2.2]paracyclophane (2), 4,7-bis(4'-dihexylaminostyryl)-12,15-bis(4' '-nitrostyryl)-[2.2]paracyclophane (3), 4,7,12-tris(4'-dihexylaminostyryl)-15-(4' '-nitrostyryl)[2.2]paracyclophane (4), 4,15-bis(4'-dihexylaminostyryl)-7,12-bis(4' '-nitrostyryl)[2.2]paracyclophane (5), and 4,12-bis(4'-dihexylaminostyryl)-7,15-bis(4' '-nitrostyryl)[2.2]paracyclophane (6). These molecules represent different combinations of bringing together distyrylbenzene chromophores containing donor and acceptor groups across a [2.2]paracyclophane (pCp) bridge. X-ray diffraction studies show that the lattice arrangements of 1 and 3 are considerably different from those of the parent chromophores 1,4-bis(4'dihexylaminostyryl)benzene (DD) and 1,4-di(4'-nitrostyryl)benzene (AA). Differences are brought about by the constraint by the pCp bridge and by virtue of chirality in the "paired" species. The absorption and emission spectra of 1-6 are also presented. Clear evidence of delocalization across the pCp structure is observed. Further, in the case of 2, 3, and 4, emission from the second excited state takes place.     

Exploring two-state reaction pathways in the photodimerization of cyclohexadiene.

The ground- (S0) and lowest triplet-state (T1) pathways associated with dimerization of cyclohexadiene to give [2+2] and [4+2] cycloadducts have been theoretically studied at the UBLYP and UB3LYP levels of theory with the 6-31G* basis set. The DFT energies were validated by CCSD(T) single-point energy calculations. These cycloaddition reactions follow stepwise mechanisms with formation of bis-allylic biradical (BB) intermediates. In the S0 ground state, the interaction between two cyclohexadiene molecules with formation of BB intermediate IN(S0) has a large activation enthalpy of 32.0 kcal mol(-1). On the other hand, C-C bond-formation in the lowest triplet state (T1) leading to BB intermediate IN(T1) has a low activation enthalpy of 5.0 kcal mol(-1), but the subsequent ring closure involves a very large activation enthalpy of 43.4 kcal mol(-1). Triplet-to-singlet intersystem crossing from IN(T1) to IN(S0) favors cyclization to give the corresponding [2+2] and [4+2] cycloadducts.     

Searching for stable, five-coordinate aquated Al(III) species. Water exchange mechanism and effect of pH

Density functional theory calculations have been performed for the water exchange mechanism of aquated Al(III). The effect of pH was considered by studying the exchange processes for [Al(H2O)6]3+ and its conjugated base, [Al(H2O)5OH]2+. Both complexes were found to exchange water in a dissociative way with activation energies (EA) of 15.9 and 10.2 kcal/mol, respectively. The influence of solvent molecules on the gas-phase cluster model was considered by the addition of up to four water molecules to the model system. The stabilizing effect of the solvent on the transition state decreases EA to 8.6 (hexa-aqua complex) and 7.6 (monohydroxo complex) kcal/mol, whereas EA for all hydroxo species is consistently significantly lower than those for the related aqua systems, which indicates a much faster water exchange rate. For the hydroxo complex, all calculated five-coordinate intermediates, nH2O.[Al(H2O)4(OH)]2+ (n = 1, 2, 3, 4, 5), are more stable than the corresponding six-coordinate reactants. Our results therefore suggest the presence of a stable five-coordinate species of aquated Al(III), namely, the [Al(H2O)4(OH)]2+ complex.     

Synthesis and reactivity of trans-Tricyclo

The first synthesis of trans-tricyclo[4.2.0.0(1,3)]oct-4-ene (1), an ethenyl bridged spirohexane, was accomplished in four steps starting from Carpino et al. gem-dichloro ketone 6. An X-ray crystal structure of 1 with one substituent was obtained to provide geometry data on this novel ring system and to confirm the stereochemical assignment of the penultimate synthetic intermediate. Tricyclo[4.2.0. 0(1,3)]oct-4-ene is surprisingly stable. It reacts with glacial acetic acid but only slowly at 145 degrees C; the products were isolated and identified. A unimolecular rearrangement takes place at elevated temperatures (165 degrees C and higher), presumably, via a biradical intermediate to afford tricyclo[4.2.0.0(1,5)]oct-3-ene (23). The structure of this 1,5-bridged bicyclo[2.1.0]pentane derivative was established by NMR and an X-ray crystal structure of its Diels-Alder adduct with isobenzofuran. Tricyclo[4.2.0.0(1, 3)]oct-4-ene equilibrates with 23, so equilibrium constants and reaction rates were measured over a 20 degrees C temperature range from 180 degrees C to 200 degrees C. The difference in the heats of formation (DeltaDeltaH degrees (f) (23 - 1)) is -2.1 kcal/mol, which is in good agreement with ab initio (HF and MP2) calculations using the 6-31G(d) basis set (-1.9 (HF) and -1.4 (MP2) kcal/mol). Computations on trans-tricyclo[4.2.0.0(1,3)]octane and spirohexane also were carried out, and the structures and energies were compared.     

Theoretical insight into stereoselective reaction mechanisms of 2,4-pentanediol-tethered ketene-olefin [2 + 2] cycloaddition

We report ab initio molecular dynamics calculations based on density functional theory performed on an intramolecular [2 + 2] cycloaddition between ketene and olefin linked with a 2,4-pentanediol (PD) tether. We find that the encounter of the ketene and olefin moieties could be prearranged in the thermal equilibrated state before the cycloaddition. The reaction mechanism is found to be stepwise, similar to that of intermolecular ketene [2 + 2] cycloadditions with ordinary alkenes. A distinct feature of the reaction pathway for a major diastereoisomer is a differential activation free energy of about 1.5 kcal/mol, including 2.8 kcal/mol as the differential activation entropy, with a transition state consisting of a flexible nine-membered ring in the olefin-PD-ketene moiety. This theoretical study provides a reasonable explanation for the strict stereocontrollability of the PD-tethered ketene-olefin cycloaddition, irrespective of reaction types or conditions.     

Favorable pendant-amino metal chelation in VX nerve agent model systems

We have performed DFT computational studies [B3LYP, 6-31+G] to obtain metal ion coordination isomers of VX-Me [MeP(O)(OMe)(SCH2CH2NMe2)], a model of two of the most lethal nerve agents: VX [MeP(O)(OEt)(SCH2CH2N(iPr)2)] and Russian-VX [MeP(O)(OCH2CHMe2)(SCH2CH2N(Et)2)]. Our calculations involved geometry optimizations of the neutral VX-Me model as well as complexes with H+, Li+, Na+, K+, Be2+, Mg2+, and Ca2+ that yielded 2-8 different stable chelation modes for each ion that involved mainly mono- and bidentate binding. Importantly, our studies revealed that the [O(P),N] bidentate binding mode, long thought to be the active mode in differentiating the hydrolytic path of VX from other nerve agents, was the most stable for all ions studied here. Binding energy depended mainly on ionic size as well as charge, with binding energies ranging from 364 kcal mol(-1) for Be2+ to 33 kcal mol(-1) for K+. Furthermore, calculated NMR shifts for VX-Me correlate to experimental values of VX.     

Ligand‐Exchange Processes on Solvated Beryllium Cations. Part III

On the basis of DFT calculations (B3LYP/6‐311+G**), the possibility to include solvent effects is considered in the investigation of the H₂O‐exchange mechanism on [Be(H₂O)₄]²⁺ within the widely used cluster approach. The smallest system in the gas phase, [Be(H₂O)₄(H₂O)]²⁺, shows the highest activation barrier of +15.6 kcal/mol, whereas the explicit addition of five H‐bonded H₂O molecules in [{Be(H₂O)₄(H₂O)}(H₂O)₅]²⁺ reduces the barrier to +13.5 kcal/mol. Single‐point calculations applying CPCM (B3LYP(CPCM:H₂O)/6‐311+G**//B3LYP/6‐311+G**) on [Be(H₂O)₄(H₂O)]²⁺ lower the barrier to +9.6 kcal/mol. Optimization of the precursor and transition state of [Be(H₂O)₄(H₂O)]²⁺ within an implicit model (B3LYP(CPCM:H₂O)/6‐311+G** or B3LYP(PCM:H₂O)/6‐311+G**) reduces the activation energy further to +8.3 kcal/mol but does not lead to any local minimum for the precursor and is, therefore, unfavorable.