Theoretical study of the gas‐phase SN2 reactions of X− with CH3OY (X,Y = Cl,Br, I)

The gas‐phase nucleophilic substitution reactions at saturated oxygen X^? + CH₃OY (X, Y = Cl, Br, I) have been investigated at the level of CCSD(T)/6‐311+G(2df,p)//B3LYP/6‐311+G(2df,p). The calculated results indicate that X^? preferably attacks oxygen atom of CH₃OY via a S_N2 pathway. The central barriers and overall barriers are respectively in good agreement with both the predictions of Marcus equation and its modification, respectively. Central barrier heights (ΔH and ΔH) correlate well with the charges (Q) of the leaving groups (Y), Wiberg bond orders (BO) and the elongation of the bonds (O? Y and O? X) in the transition structures. © 2007 Wiley Periodicals, Inc. Int J Quantum Chem, 2007     

Schwingungsspektren der Clusterverbindungen (M6X)X · 8 H2O,M = Nb,Ta; Xi = CI,Br; Xa = CI,Br, I

Vibrational Spectra of the Cluster Compounds (M₆X₁₂ⁱ) · 8H₂O, M = Nb, Ta; Xⁱ = Cl, Br; X^a = Cl, Br, I IR and, for the first time, Raman spectra at 80 K of the cluster compounds (M₆X)X · 8H₂O; M = Nb, Ta; Xⁱ = Cl, Br; X^a = Cl, Br, I, have been recorded, characterized by typical frequencies of the (M₆X) unit, which are only slightly influenced by the terminal X^a ligands. The most intense line with the depolarisation ≈? 0.2 in all Raman spectra is caused by inphase movement of all atoms and assigned to the symmetric metal-metal vibration v₁, observed for the clusters (Nb₆Cl) at 233–234, for (Nb₆Br) at 186–187, for (Ta₆Cl) at 199–203, and for (Ta₆Br) at 176–179 cm^?1. The IR spectra exhibit in the same series intense bands at 233, 204, 207, and 179 cm^?1, assigned to the antisymmetric metal-metal vibration. The metal-metal frequencies are significantly higher than discussed before. The tantalum clusters show on excitation with the krypton line 647.1 nm in the region of a d–d transition at 645 nm a resonance Raman effect with series of overtones and combination bands. In case of (Ta₆Br) another polarisized band is observed at 229 cm^?1 and assigned to the Ta? Brⁱ vibration v₂. From the progressions of v₁ and v₂ anharmonicity constants of about ?3 cm^?1 are calculated indicating a strong distortion of the potential curves.     

The haloacetyl ions; a thermochemical study

The haloacetyl ions [XCH₂CO⁺], X = Cl, Br, I, have been produced by the dissociative ionization of selected precursor molecules and identified via their fragmentation characteristics. Their heats of formation, Δ, were 708 ± 8, 750 ± 25 and 784 ± 10 kJ mol^?1, respectively, all appreciably above ΔH for [CH₃CO⁺], 653 kJ mol^?1. The effect of halogen substitution α to carbonyl is discussed for neutral molecules and their molecular ions.     

Theoretical investigations on thallium halides: Relativistic and electron correlation effects in T1X and T1X3 compounds (XF,C1, Br,and I)

Relativistic and electron correlation effects in thallium halides TlX and TlX₃ (X?F, Cl, Br, and I) are investigated by extensive ab initio configuration interaction calculations. Spin–orbit coupling is included at the Hartree–Fock level for the diatomic TlBr and TlI. At the best level of treatment of electron correlation (quadratic configuration interaction), the calculated molecular properties are in good agreement with experimental results, i.e., for the diatomic thallium halides deviations from experimental values are <0.06 Å for bond distances, <0.14 mdyn/Å for force constants, <35 kJ/mol for dissociation energies, and <0.3 D for dipole moments. The convergence of the Møller–Plesset series up to the fourth order is discussed. Two alternative structures of TlI₃ are compared. At the Møller–Plesset level of theory, the trigonal planar structure with thallium in the oxidation state + 3 is the preferred gas phase arrangement compared with the bent arrangement containing a linear I unit and thallium in the oxidation state + 1, the difference being ca. 95 kJ/mol. Vibrational frequencies are predicted for all trigonal planar thallium(III) halides. © 1993 John Wiley & Sons, Inc.     

Zweiwertiges Samarium: AISmX5 (AI = K,Rb; X = Cl,Br, I)

Divalent Samarium: A^ISm X₅ (A^I = K, Rb; X = Cl, Br, I) Ternary halides with divalent Samarium ASm₂X₅ were prepared and investigated by X-ray techniques. The paramagnetic susceptibility of Sm²⁺ has been measured with a Faraday balance and calculated theoretically.     

Beiträge zur Chemie der Elemente Niob und Tantal. 84 Die Niob- und Tantalkomplexe [Me6X] X · n H2O mit Me = Nb,Ta; Xi = Cl,Br; Xa = Cl,Br, J

On the chemistry of the elements niobium and tantalum. 84. The niobium and tantalum complexes [Me₆X]X · n H₂O with Me = Nb, Ta; X¹ = Cl, Br; X^a = Cl, Br, J The known and unknown compounds mentioned in the title were prepared. In this group of compounds four different crystal structures (A, B, C, D) occur. Lattice constants are given of the six compounds with structure C which crystallize in the hexagonal system and are isotypic with Ba₂[Nb₆Cl₁₂]Cl₆. Regarding the IR-spectra and the thermal behaviour, possible principles of structure are discussed.     

Darstellung,Kristallstruktur und spektroskopische Eigenschaften der Clusteranionen [(Mo6Br)X]2− mit Xa = F,Cl, Br,I

Synthesis, Crystal Structure and Spectroscopic Properties of the Cluster Anions [(Mo₆Br )X ]^2? with X^a = F, Cl, Br, I The tetrabutylammonium (TBA), tetraphenylphosphonium (TPP) and tetraphenylarsonium (TPAs) salts of the octa-μ₃-bromo-hexahalogeno-octahedro-hexamolybdate(2?) anions [(Mo₆Br)X]^2? (X^a = F, Cl, Br, I) are synthesized from solutions of the free acids H₂[(Mo₆Br)X] · 8 H₂O with X^a = Cl, Br, I. The crystal structures show systematic stretchings in the Mo? Mo bond length and a slight compression of the Brⁱ₈ cube in the F^a to I^a series. The cations do not change much. The i.r. and Raman spectra show at 10 K almost constant frequencies of the (Mo₆Brⁱ₈) cluster vibrations, whereas all modes with X^a ligand contribution are characteristically shifted. The most important bands are assigned by polarization measurements and the force constants are derived from normal coordinate analysis. The ⁹⁵Mo nmr signals are shifted to lower field with increasing electronegativity of the X^a ligands. The fluorine compound shows a sharp ¹⁹F nmr singlet at ?184.5 ppm.     

Stability of Complex Metal Bromides in the gas phase and in toluene

The equilibrium constants of the reactions MBr₂(s) + Al₂Br₆(sln) ? MAl₂Br₈(sln) M = Cr, Mn, Co, Ni, Zn, Cd have been measured at 298 K in toluene. Ni: 0.017 ± 0.0024, Co: 0.54 ± 0.07, Zn: 1.5 ± 0.2, Mn: 2.1 ± 0, 7, Cr: 2.2 ± 1, Cd: 7 ± 5. They are compared with literature values of the equilibrium constants of analogous reactions in the gas phase MX₂(s) + Al₂X₆(g) ? MAl₂X₈(g), X = Cl, Br. For CoAl₂Br₈(sln) the temperature dependence of the equilibrium constant yielded Δ_fH = ?9.4 ± 1 kJ mol^?1 and Δ_fS = ?39.5 ± 3 J mol^?1 K^?1 while literature values for CoAl₂Br₈(g) are Δ_fH = 42.4 ± 2 kJ mol^?1 and Δ_fS = 42.9 ± 2 J mol^?1 K^?1. The solubility of Al₂Br₆ in toluene as well as its enthalpy of dissolution have been measured in order to evaluate ΔH° and ΔS° of the solvation of Al₂Br₆(g) and CoAl₂Br₈(g) in toluene by a thermodynamic cycle. Solvation of Al₂Br₆(g): ΔH = ?72.7 ± 1 kJ mol^?1, ΔS = ?139.6 ± 4 J mol^?1 K^?1, solvation of CoAl₂Br₈(g): ΔH = ?124.5 ± 4kJ mol^?1, ΔS = ?222 ± 9J mol^?1 K^?1. Thus, CoAl₂Br₈ interacts more strongly with the solvent toluene than Al₂Br₆ does.     

Gas‐phase intramolecular anion rearrangements of some trimethylsilyl‐containing systems revisited. A theoretical approach

Ab initio calculations at the CCSD(T)/6‐311++G(2d,p)//B3LYP/6‐311++G(d,p) level of theory have been carried out for three prototypical rearrangement processes of organosilicon anion systems. The first two are reactions of enolate ions which involve oxygen–silicon bond formation via three‐ and four‐membered states, respectively. The overall reactions are: The ΔG (reaction) values for the two processes are +175 and +51 kJ mol^?1, with maximum barriers (to the highest transition state) of +55 and +159 kJ mol^?1, respectively. The third studied process is the following: (CH₃O)C(?CH₂)Si(CH₃)₂CH → (CH₃)₂(C₂H₅)Si^? + CH₂CO, a process involving an S_Ni reaction between ‐CH and CH₃O‐ followed by silicon–carbon bond cleavage. The reaction is favourable [ΔG(reaction) = ?39 kJ mol^?1] with the barrier for the S_Ni process being 175 kJ mol^?1. The previous experimental and the current theoretical data are complementary and in agreement. Copyright © 2009 John Wiley & Sons, Ltd.     

Schwingungsspektren und Normalkoordinatenanalysen der 92Mo-, 100Mo-, 35Cl- und 37Cl-Isotopomeren der Clusteranionen [(Mo6X)Y]2−; Xi = Cl,Br; Ya = F,Cl, Br,I

Vibrational Spectra and Normal Coordinate Analysis of ⁹²Mo, ¹⁰⁰Mo, ³⁵Cl, and ³⁷Cl Isotopomers of the Cluster Anions [(Mo₆X )Y ]^2?; Xⁱ = Cl, Br; Y^a = F, Cl, Br, I The tetrabutylammonium (TBA) salts of the octa-μ₃-halogeno-hexahalogeno-octahedro-hexamolybdate(2 –) anions [(Mo₆X)Y]^2?; Xⁱ = Cl, Br; Y^a = F, Cl, Br, I; have been synthesized using ⁹²Mo, ¹⁰⁰Mo, ³⁵Cl, and ³⁷Cl. The 10 K IR and Raman spectra reveal significant frequency shifts due to the isotopic labelling of the Mo₆ cage, the inner sphere halides X₈ⁱ or the outer sphere ligands Y, respectively. The normal coordinate analysis yields (Mo? Mo) valence force constants of about 1.3 to 1.5 mdyn/Å. For the μ₃-bonded halogenes Clⁱ and Brⁱ valence force constants of 1.1 resp. 1.0 mdyn/Å are calculated. The values for (Mo? Y^a) bonds are found in the usual halide range. The observed isotopic shifts are verified very well by the calculations, allowing detailed assignment of the IR and Raman spectra of these compounds for the first time.     

Kinetic study of the interaction of adenosine 5′‐monophosphate with diaqua (2‐hydrazinopyridine) palladium(II) in aqueous medium

The interaction of the palladium(II) complex [Pd(hzpy)(H₂O)₂]²⁺, where hzpy is 2‐hydrazinopyridine, with purine nucleoside adenosine 5′‐monophosphate (5′‐AMP) was studied kinetically under pseudo‐first‐order conditions, using stopped‐flow techniques. The reaction was found to take place in two consecutive reaction steps, which are both dependent on the actual 5′‐AMP concentration. The activation parameters for the two reaction steps, i.e. ΔH = 32 ±2 kJ mol^?1, ΔS = ?168 ±7 J K^?1 mol^?1, and ΔH = 28 ± 1 kJ mol^?1, ΔS = ?126 ± 5 J K^?1 mol^?1, respectively, were evaluated and suggested an associative mode of activation for both substitution processes. The stability constants and the associated speciation diagram of the complexes were also determined potentiometrically. The isolated solid complex was characterized by C, H, and N elemental analyses, IR, magnetic, and molar conductance measurements. © 2009 Wiley Periodicals, Inc. Int J Chem Kinet 42: 132–142, 2010     

Variable-Temperature and Variable-Pressure Kinetic and Equilibrium studies of formation and dissociation of [V(H2O)5NCS]2+

The kinetics of formation and dissociation of [V(H₂O)₅NCS]²⁺ have been studied, as a function of excess metal-ion concentration, temperature, and pressure, by the stopped-flow technique. The thermodynamic stability of the complex was also determined spectrophotometrically. The kinetic and equilibrium data were submitted to a combined analysis. The rate constants and activation parameters for the formation (f) and dissociation (r) of the complex are: k/M ^?1 · S^?1 = 126.4, k/s^?1 = 0.82; ΔH /kJ · mol^?1 = 49.1, ΔH/kJ · mol^?1 = 60.6; ΔS/ J·K^?1·mol^?1= ?39.8, ΔSJ·K^?1·mol^?1 = ?43.4; ΔV/cm³·mol^?1 = ?9.4, and ΔV/cm³ · mol^?1 =?17.9. The equilibrium constant for the formation of the monoisothiocynato complex is K²⁹⁸/M ^?1 = 152.9, and the enthalpy and entropy of reaction are ΔH⁰/kJ · mol^?1 = ? 11.4 and ΔS⁰/J. K^?1mol^?1 = +3.6. The reaction volume is ΔV⁰/cm³· mol^?1 = +8.5. The activation parameters for the complex-formation step are similar to those for the water exchange on [V(H₂O)₆]³⁺ obtained previously by NMR techniques. The activation volumes for the two processes are consistent with an associative interchange, I_a, mechanism.     

Photoelektronen-Spektren und Moleküleigenschaften. 132 [1, 2]. Trifluormethylsulfan und Derivate F3CSX (X  CF3, Cl,Br, I)

Photoelectron Spectra and Molecular Properties. 132. Trifluoromethylsulfane and Derivatives F₃CSX (X ? CF₃, Cl, Br, I) The He(I) photoelectron spectra of trifluoromethylsulfane F₃CSH and its derivatives F₃CSX (X ? CF₃, Cl, Br, I) are assigned by Koopmans' correlations, IE = ?ε, with MNDO eigenvalues, by radical cation state comparison and based on resolved vibrational fine structures, which can' be discussed by MNDO FORCE calculations. The spin/orbit splitting in F₃CSI can be approximated by additional ITEREX-85 calculations. Gasphase thermolysis of the trifluoromethylhalogensulfanes F₃CSX at 10^?4 mbar yields decomposition temperatures, which decrease from X ? Cl to I, and as fragmentation products of presumably radical intermediates, in addition to the respective halogens X₂ and F₂C?S, also F₃CX as well as S₂ and CS₂ (X ?Cl, Br) are PE spectroscopically detected.     

Studies on the composition and kinetics of chromium(III)-alanine system

Kinetics of the complex formation of chromium(III) with alanine in aqueous medium has been studied at 45, 50, and 55°C, pH 3.3–4.4, and μ = 1 M (KNO₃). Under pseudo first-order conditions the observed rate constant (k_obs) was found to follow the rate equation: Values of the rate parameters (k_an, k, K_IP, and K) were calculated. Activation parameters for anation rate constants, ΔH^≠(k_an) = 25 ± 1 kJ mol^?1, ΔH^≠(k) = 91 ± 3 kJ mol^?1, and ΔS^≠(k_an) = ?244 ± 3 JK^?1 mol^?1, ΔS^≠(k) = ?30 ± 10 JK^?1 mol^?1 are indicative of an (I_a) mechanism for k_an and (I_d) mechanism for k routes (‥substrate Cr(H₂O) is involved in the k route whereas Cr(H₂O)₅OH²⁺ is involved in k′ route). Thermodynamic parameters for ion-pair formation constants are found to be ΔH°(K_IP) = 12 ± 1 kJ mol^?1, ΔH°(K) = ?13 ± 3 kJ mol^?1 and ΔS°(K_IP) = 47 ± 2 JK^?1 mol^?1, and ΔS°(K) = 20 ± 9 JK^?1 mol^?1.     

Properties and reactivity in groups of the periodic system: Ion-molecule reactions CH3X + CH3X+· (X = F through At)

The reactions indicated in the title have been studied in terms of direct processes and complex formation. Quantum-chemical methods have been applied to the passage of an acid (H⁺, CH, X⁺) from CH₃X^+· to CH₃X, and the abstraction of a radical (H^· CH, X^·) from CH₃X by CH₃X^+·. It has been shown that a complex represented by a dimer of a methyl-halide radical cation, (CH₃X), with a two-center three-electron bond X? X, has fairly high stability. These investigations were based on non-empirical quantum-chemical calculations, the results being systematically compared with experimental determinations. Some calculations included all electrons (X=F, Cl, Br), others were based on relativistic pseudopotentials (X=F through At). The two sets of calculations agree qualitatively with each other and with experimental observations.     

Ab initio molecular orbital studies of nonidentity allyl transfer reactions

Ab initio molecular orbital (MO) calculations are carried out on the nonidentity allyl transfer processes, X^? + CH₂CHCH₂Y ? CH₂CHCH₂ X + Y^?, with X^? = H, F, and Cl and Y = H, NH₂, OH, F, PH₂, SH, and Cl. The Marcus equation applies well to the allyl transfer reactions. The transition state (TS) position along the reaction coordinate and the TS structure are strongly influenced by the thermodynamic driving force, whereas the TS looseness is originated from the intrinsic barrier. The intrinsic barrier, ΔE, looseness, %L?, and absolute asymmetry, %AS?, are well correlated with the percentage bond elongation, %CY? = [(d ? d)/d] × 100 and/or %CX?. The %CY? and the bond orders indicate that a stronger nucleophile and/or a stronger nucleofuge (or a better leaving group) leads to an earlier TS on the reaction coordinate with a lesser degree of bond making as well as bond breaking. These are consistent with the Bell-Evans-Polanyi principle and the Leffler-Hammond postulate. © 1995 by John Wiley & Sons, Inc.     

MP2, DFT‐D,and PCM study of the HMB–TCNE complex: Thermodynamics,electric properties,and solvent effects

Geometry, thermodynamic, and electric properties of the π‐EDA complex between hexamethylbenzene (HMB) and tetracyanoethylene (TCNE) are investigated at the MP2/6‐31G* and, partly, DFT‐D/6‐31G* levels. Solvent effects on the properties are evaluated using the PCM model. Fully optimized HMB–TCNE geometry in gas phase is a stacking complex with an interplanar distance 2.87 × 10^?10 m and the corresponding BSSE corrected interaction energy is ?51.3 kJ mol^?1. As expected, the interplanar distance is much shorter in comparison with HF and DFT results. However the crystal structures of both (HMB)₂–TCNE and HMB–TCNE complexes have interplanar distances somewhat larger (3.18 and 3.28 × 10^?10 m, respectively) than our MP2 gas phase value. Our estimate of the distance in CCl₄ on the basis of PCM solvent effect study is also larger (3.06–3.16 × 10^?10 m). The calculated enthalpy, entropy, Gibbs energy, and equilibrium constant of HMB–TCNE complex formation in gas phase are: ΔH⁰ = ?61.59 kJ mol^?1, ΔS = ?143 J mol^?1 K^?1, ΔG⁰ = ?18.97 kJ mol^?1, and K = 2,100 dm³ mol^?1. Experimental data, however, measured in CCl₄ are significantly lower: ΔH⁰ = ?34 kJ mol^?1, ΔS = ?70.4 J mol^?1 K^?1, ΔG⁰ = ?13.01 kJ mol^?1, and K = 190 dm³ mol^?1. The differences are caused by solvation effects which stabilize more the isolated components than the complex. The total solvent destabilization of Gibbs energy of the complex relatively to that of components is equal to 5.9 kJ mol^?1 which is very close to our PCM value 6.5 kJ mol^?1. MP2/6‐31G* dipole moment and polarizabilities are in reasonable agreement with experiment (3.56 D versus 2.8 D for dipole moment). The difference here is due to solvent effect which enlarges interplanar distance and thus decreases dipole moment value. The MP2/6‐31G* study supplemented by DFT‐D parameterization for enthalpy calculation, and by the PCM approach to include solvent effect seems to be proper tools to elucidate the properties of π‐EDA complexes. © 2008 Wiley Periodicals, Inc. Int J Quantum Chem, 2008     

Tetramethylcyclotetraarsoxane Bridged Copper(I) Halides with Porous Sheet and Framework Structures [CuX{cyclo-(MeAsO)4}] (X=Cl,Br, I) and [Cu3X3{cyclo-(MeAsO)4}2] (X=Cl,Br)

Open sheet and framework structures [CuX{cyclo-(MeAsO)₄}] (X=Cl, Br, I) 1 – 3 and [Cu₃X₃{cyclo-(MeAsO)₄}₂] (X=Cl, Br) 4 and 5 may be prepared by self-assembly from CuX and methylcycloarsoxane (MeAsO)_n in acetonitrile solution. 1 – 3 exhibit 4⁴ nets in which (CuX)₂ units are connected through μ-1 _KAs¹ : 2 _KAs³ coordinated (MeAsO)₄ ligands into large 28-membered rings. In contrast, adjacent [CuX] chains in 4 and 5 are connected into sheets by μ₄-K⁴ As coordinated (MeAsO)₄ building blocks, with μ-1 _KAs¹ : 2 _KAs³ bridging of these layers by independent (MeAsO)₄ cyclotetramers leading to the generation of a porous framework structure. 1 – 5 were characterised by X-ray structural analysis.     

Density functional study of S(N) 2 substitution reactions for CH(3) Cl + CX(1) X(2•-) (X(1) X(2) = HH, HF, HCl, HBr, HI, FF, ClCl, BrBr, and II)

A systematic investigation on the S_N2 displacement reactions of nine carbene radical anions toward the substrate CH₃Cl has been theoretically carried out using the popular density functional theory functional BHandHLYP level with different basis sets 6‐31+G (d, p)/relativistic effective core potential (RECP), 6‐311++G (d, p)/RECP, and aug‐cc‐pVTZ/RECP. The studied models are CX¹X^2?? + CH₃Cl → X²X¹CH₃C^? + Cl^?, with CX¹X^2?? = CH₂^??, CHF^??, CHCl^??, CHBr^??, CHI^??, CF₂^??, CCl₂^??, CBr₂^??, and CI₂^??. The main results are proposed as follows: (a) Based on natural bond orbital (NBO), proton affinity (PA), and ionization energy (IE) analysis, reactant CH₂^?? should be a strongest base among the anion‐containing species (CX¹X^2??) and so more favorable nucleophile. (b) Regardless of frontside attacking pathway or backside one, the S_N2 reaction starts at an identical precomplex whose formation with no barrier. (c) The back‐S_N2 pathway is much more preferred than the front‐S_N2 one in terms of the energy gaps [ΔE(front)?ΔE(back)], steric demand, NBO population analysis. Thus, the back‐S_N2 reaction was discussed in detail. On the one hand, based on the energy barriers (ΔE and ΔE) analysis, we have strongly affirmed that the stabilization of back attacking transition states (b‐TSs) presents increase in the order: b‐TS‐CI₂ < b‐TS‐CBr₂ < b‐TS‐CCl₂ < b‐TS‐CHI < b‐TS‐CHBr < b‐TS‐CHCl < b‐TS‐CF₂ < b‐TS‐CHF < b‐TS‐CH₂. On the other hand, depended on discussions of the correlations of ΔE with influence factors (PA, IE, bond order, and ΔE), we have explored how and to what extent they affect the reactions. Moreover, we have predicted that the less size of substitution (α‐atom) required for the gas‐phase reaction with α‐nucleophile is related to the α‐effect and estimated that the reaction with the stronger PA nucleophile, holding the lighter substituted atom, corresponds to the greater exothermicity given out from reactants to products. © 2012 Wiley Periodicals, Inc. J Comput Chem, 2012     

Tris (methylidene)-cyclopropane (“[3]Radialene”) Part 1. Enthalpy of formation and strain energy

The enthalpy of formation of tris(methylidene)-cyclopropane (“[3]radialene”, 1 ) has been determined as ΔH = 396 ± 12 kJ mol^?1 from three fragmentation reactions of its molecular ion 1 ⁺ formed from 1 by photoionisation using synchrotron radiation. Comparative electron impact measurements using conventional mass spectrometry were also performed. A treatment of the latter data is described which leads to satisfactory agreement with the photoionization data. The experimental value of ΔH( 1 ) is compared with various theoretical estimates. The strain energy of 1 is calculated to be 226.3 kJ mol^?1. Linear extrapolation of this quantity from the increase of strain in passing from cyclopropane to methylidenecyclopropane yields 282.4 kJ mol^?1. The discrepancy between these values, already predicted by Dewar and Baird ten years ago from theoretical calculations, is discussed on the basis of maximum overlap considerations. The enthalpy of formation of bis(methylidene)cyclopropane is predicted to be ΔH= 309 kJ mol^?1.