Tímea
Szűcs
and
Gábor
Czakó
*
MTA-SZTE Lendület Computational Reaction Dynamics Research Group, Interdisciplinary Excellence Centre and Department of Physical Chemistry and Materials Science, Institute of Chemistry, University of Szeged, Rerrich Béla tér 1, Szeged H-6720, Hungary. E-mail: gczako@chem.u-szeged.hu
First published on 3rd August 2022
This electronic structure study reveals four exothermic and two endothermic reaction pathways of the F + CH3NH2 system: the barrierless hydrogen abstraction from the methyl/amino group (HF + CH2NH2/CH3NH), amino/methyl substitution (NH2 + CH3F and CH3 + NH2F) and hydrogen substitution from the two functional groups (H + CH2FNH2/CH3NHF). The benchmark classical and adiabatic energies are obtained using a high-accuracy composite ab initio approach, where the gold-standard explicitly-correlated coupled cluster method (CCSD(T)-F12b) is applied with the correlation-consistent polarized valence quintuple-zeta F12 basis set (cc-pV5Z-F12) and further additive energy contributions. Considering indispensable post-(T) correlation, core correlation, scalar relativistic, spin–orbit and harmonic zero-point energy corrections, the obtained global minimum of the potential energy surface is the post-reaction CH2NH2⋯HF complex in the product channel. Although each substitution pathway has a high barrier, the energies of amino-substitution and methyl-hydrogen-substitution products are below the energy of the reactants, as well as the submerged-barrier hydrogen-abstraction pathways.
In the present study we focus on the F + CH3NH2 system following the early experiments18,19 on the F + CH3ND2 reaction, which measured the HF/DF vibrational and rotational distributions and the theoretical study of Schaefer and co-workers,7 which characterized the two hydrogen-abstraction pathways. What comes after the previous work on the title reaction? First, it is important to note that the previous computations only considered the H-abstraction channels, whereas our recent studies on the F/Cl/Br/I/OH + C2H620,21 and Cl + CH3NH2/CH3CN22,23 reactions revealed several other channels, such as hydrogen and methyl substitution, for the reactions of C2H6, and the picture is even more complex in the cases of CH3NH2 and CH3CN. Thus, in the present study, we plan to characterize all the possible, chemically relevant pathways of the title reaction, thereby providing new qualitative insights into the mechanisms of the F + CH3NH2 process. Furthermore, knowing the challenging electronic structure of the reactions of the fluorine atom, we plan to use the highest technically feasible level of theory to achieve sub-chemical accuracy for the relative energies of the stationary points. In 2013 Schaefer and co-workers7 used the standard CCSD(T) method with the aug-cc-pVnZ [n = 2(D), 3(T), 4(Q)] basis sets; however, this previous work did not consider accelerating the basis-set convergence using the explicitly-correlated CCSD(T)-F12b method, electron correlation beyond CCSD(T), and the effects of core correlation, scalar relativity, and spin–orbit coupling. In the present study, we take all of the above-mentioned corrections into account, thereby anchoring the relative energies of the stationary points of the title reaction. These new benchmark data will guide future PES developments and reaction-dynamics simulations, whose results may be directly compared with experiments. In Section II we describe the details of the composite ab initio computations; the results are presented and discussed in Section III; and the paper ends with our summary and conclusions in Section IV.
The most accurate benchmark classical relative energies are obtained in this work using the following expression:
ΔEclassical = CCSD(T)-F12b/cc-pV5Z-F12 + δ[CCSDT] + δ[CCSDT(Q)] + Δcore + Δrel + ΔSO, | (1) |
To acquire the post-(T) correlation energy contributions, unrestricted CCSD(T),28 CCSDT29 and CCSDT(Q)30 computations – using the unrestricted Hartree–Fock (UHF) reference and the aug-cc-pVDZ basis set – are performed using the MRCC program suite31,32 interfaced to MOLPRO.33 The corrections are calculated as follows:
δ[CCSDT] = ΔE(CCSDT/aug-cc-pVDZ) − ΔE(CCSD(T)/aug-cc-pVDZ), | (2) |
δ[CCSDT(Q)] = ΔE(CCSDT(Q)/aug-cc-pVDZ) − ΔE(CCSDT/aug-cc-pVDZ). | (3) |
Δcore = ΔE(AE-CCSD(T)-F12b/cc-pCVTZ-F12) − ΔE(FC-CCSD(T)-F12b/cc-pCVTZ-F12). | (4) |
Δrel = ΔE(DK-AE-CCSD(T)/aug-cc-pwCVTZ-DK) − ΔE(AE-CCSD(T)/aug-cc-pwCVTZ). | (5) |
The interacting-states approach37 is used with the Davidson-corrected38 all-electron multi-reference configuration interaction39 (MRCI+Q) method to determine the spin–orbit (SO) coupling effect. The aug-cc-pwCVTZ basis set40 is used and an active space of 5 electrons in the 3 spatial 2p-like fluorine orbitals is applied. During the SO computations the diagonal elements of the 6 × 6 SO matrix are replaced by the Davidson-corrected MRCI energies and the SO eigenstates are obtained by diagonalizing this matrix. Subtracting the non-SO ground-state (non-SO1) energy from the SO ground-state (SO1) energy gives the ΔSO correction:
ΔSO = SO1(MRCI+Q/aug-cc-pwCVTZ) − non-SO1(MRCI+Q/aug-cc-pwCVTZ). | (6) |
Considering the zero-point energy contributions (ΔZPE) obtained at the CCSD(T)-F12b/aug-cc-pVTZ level of theory, the vibrationally adiabatic relative energies are determined as follows:
ΔEadiabatic = CCSD(T)-F12b/cc-pV5Z-F12 + δ[CCSDT] + δ[CCSDT(Q)] + Δcore + Δrel + ΔSO + ΔZPE. | (7) |
The computations detailed above are accomplished using the MOLPRO program package.33
Stationary points | CCSD(T)-F12b | δ[T]b | δ[(Q)]c | Δ core | Δ rel | Δ SO | Classicalg | Δ ZPE | Adiabatici |
---|---|---|---|---|---|---|---|---|---|
cc-pV5Z-F12a | |||||||||
a CCSD(T)-F12b/cc-pV5Z-F12 relative energies. b CCSDT – CCSD(T) obtained using the aug-cc-pVDZ basis set. c CCSDT(Q) – CCSDT obtained using the aug-cc-pVDZ basis set. d Core correlation correction obtained as the difference between all-electron and frozen-core CCSD(T)-F12b/cc-pCVTZ-F12 relative energies. e Scalar relativistic effect obtained as the difference between DK-AE-CCSD(T)/aug-cc-pwCVTZ-DK and AE-CCSD(T)/aug-cc-pwCVTZ relative energies. f Spin–orbit (SO) corrections obtained as the difference between the SO and non-SO ground-state MRCI+Q/aug-cc-pwCVTZ relative energies. g Benchmark classical relative energies obtained as CCSD(T)-F12b/cc-pV5Z-F12 relative energies + δ[T] (b) + δ[(Q)] (c) + Δcore (d) + Δrel (e) + ΔSO (f). h Zero-point energy (ZPE) corrections obtained at CCSD(T)-F12b/aug-cc-pVTZ. i Benchmark adiabatic relative energies obtained as classical relative energies (g) + ΔZPE (h). | |||||||||
PREMIN | −20.61 | −0.20 | −0.46 | −0.08 | +0.11 | +0.37 | −20.87 | +1.15 | −19.72 |
CH3 HA TS′ | −9.24 | −0.66 | −0.52 | −0.12 | +0.20 | +0.34 | −9.99 | −1.65 | −11.64 |
CH3 HA TS′′ | −11.39 | −0.54 | −0.46 | −0.14 | +0.14 | +0.36 | −12.04 | −2.54 | −14.58 |
NH2 HA TS | −19.31 | −0.40 | −0.44 | −0.08 | +0.10 | +0.36 | −19.77 | +0.07 | −19.70 |
AS TS W | 18.37 | −0.60 | −0.62 | +0.47 | +0.01 | +0.36 | 17.98 | −1.32 | 16.66 |
AS TS FS | 37.72 | −1.08 | −0.69 | +0.53 | +0.00 | +0.37 | 36.84 | −1.32 | 35.52 |
MS TS | 26.77 | −0.75 | −0.63 | +0.49 | −0.04 | +0.37 | 26.22 | −2.24 | 23.98 |
CH3 HS TS | 18.30 | −0.42 | −0.58 | +0.12 | +0.11 | +0.37 | 17.90 | −3.60 | 14.30 |
NH2 HS TS | 37.53 | −0.28 | −0.52 | +0.33 | +0.01 | +0.37 | 37.43 | −3.79 | 33.64 |
CH3 HA MIN′ | −48.51 | −0.08 | −0.22 | −0.22 | +0.20 | +0.37 | −48.46 | −0.73 | −49.19 |
CH3 HA MIN′′ | −50.80 | −0.06 | −0.19 | −0.19 | +0.20 | +0.37 | −50.67 | −0.38 | −51.05 |
NH2 HA MIN | −45.50 | −0.16 | −0.12 | −0.01 | +0.13 | +0.37 | −45.30 | −0.99 | −46.29 |
AS MIN W | −25.42 | −0.17 | −0.05 | +0.30 | +0.07 | +0.37 | −24.90 | −2.91 | −27.81 |
AS MIN FS | −26.40 | −0.17 | −0.05 | +0.28 | +0.08 | +0.37 | −25.90 | −2.24 | −28.15 |
MS MIN | 16.03 | −0.18 | −0.25 | +0.28 | +0.04 | +0.37 | 16.29 | −3.42 | 12.87 |
CH3 HS MIN | −17.69 | +0.07 | −0.19 | +0.06 | +0.17 | +0.37 | −17.22 | −3.68 | −20.90 |
NH2 HS MIN | 33.10 | +0.00 | −0.34 | +0.23 | +0.05 | +0.37 | 33.41 | −4.68 | 28.73 |
CH3NHF + H | 33.47 | +0.00 | −0.34 | +0.23 | +0.05 | +0.37 | 33.78 | −5.00 | 28.77 |
NH2F + CH3 | 17.91 | −0.16 | −0.24 | +0.28 | +0.04 | +0.37 | 18.19 | −4.25 | 13.94 |
CH2FNH2 + H | −17.42 | +0.07 | −0.19 | +0.06 | +0.17 | +0.37 | −16.94 | −3.89 | −20.83 |
CH3F + NH2 | −23.47 | −0.17 | −0.04 | +0.29 | +0.07 | +0.37 | −22.94 | −3.53 | −26.48 |
HF + CH3NH | −33.92 | −0.16 | −0.11 | +0.07 | +0.10 | +0.37 | −33.65 | −3.59 | −37.24 |
HF + CH2NH2 | −41.38 | −0.07 | −0.18 | −0.19 | +0.20 | +0.37 | −41.26 | −2.86 | −44.12 |
CH4 + NHF | 3.77 | −0.38 | −0.19 | +0.35 | −0.05 | +0.37 | 3.87 | −3.84 | 0.03 |
NH3 + CH2F | −29.96 | −0.15 | −0.11 | +0.07 | +0.15 | +0.36 | −29.65 | −3.12 | −32.77 |
The most exothermic mechanisms are the hydrogen abstraction (HA) reactions, with ΔE(ΔH0) = −41.26(−44.12) kcal mol−1 in the case of hydrogen transfer from the methyl group and ΔE(ΔH0) = −33.65(−37.24) kcal mol−1 for HA from the amino group. These are followed by the substitution pathways as can be seen in Fig. 1. We found that the CH3 HA MIN′′ structure in the product channel is the global minimum of the PES, where the formed HF binds to the nitrogen atom through a 1.409 Å N⋯H distance. We present two transition states in the methyl hydrogen-abstraction channel, where the CH3 HA TS′ has very similar geometrical and energetic parameters to the structure reported by Feng et al.,7 but the classical(adiabatic) relative energy is more negative by 2.05(2.94) kcal mol−1 in the case of TS′′. The TS′ and TS′′ structures differ in bond lengths by a few hundredths of an ångström and in angles, mainly the plane of the C–N–H atoms relative to the plane of the C–H–F atoms. In the entrance channel a reactant-like pre-reaction complex with a substantial dissociation energy (De(D0) = 20.87(19.72) kcal mol−1) is obtained with a C–N–F angle of 102.8°.
Amino substitution (AS) and methyl-hydrogen substitution (CH3 HS) are thermodynamically favored like the hydrogen abstractions; however, these substitutions are kinetically not preferred. We could identify front-side attack (FS) and Walden-inversion (W) mechanisms in the case of amino-substitution, where the former has a higher barrier (with 18.86 kcal mol−1) as expected. The classical relative energies of the AS MIN FS and AS MIN W structures in the product channel differ by 1 kcal mol−1, which is in accordance with the very similar lengths of the C–N (3.366/3.185 Å) and C–F (1.392/1.388 Å) bonds. The dipole-polarized-atom force-stabilized CH3 HS MIN complex has the lowest dissociation energy, De(D0) = 0.28(0.07) kcal mol−1, among the product-like complexes.
The most endothermic pathway is hydrogen substitution from the amino group, and it has the highest barrier. The exit-channel minimum of this process is quite shallow, like in the other HS pathway. The methyl substitution is endothermic as well (ΔE(ΔH0) = 18.19(13.94) kcal mol−1), but it has a lower relative energy than the above-mentioned NH2 HS channel (ΔE(ΔH0) = 33.78(28.77) kcal mol−1).
Stationary points | MP2 | CCSD(T)-F12b | ||||||
---|---|---|---|---|---|---|---|---|
aug-cc-pVDZ | aug-cc-pVDZ | aug-cc-pVTZ | aug-cc-pVQZ | cc-pVDZ-F12 | cc-pVTZ-F12 | cc-pVQZ-F12 | cc-pV5Z-F12 | |
PREMIN | −27.63 | −21.31 | −20.63 | −20.69 | −20.39 | −20.26 | −20.54 | −20.61 |
CH3 HA TS′ | −7.33 | −9.14 | −9.39 | −9.38 | −8.07 | −9.00 | −9.21 | −9.24 |
CH3 HA TS′′ | −11.59 | −11.45 | −11.44 | −11.52 | −10.25 | −11.10 | −11.33 | −11.39 |
NH2 HA TS | −17.30 | −18.61 | −19.38 | −19.40 | −18.04 | −19.07 | −19.27 | −19.31 |
AS TS W | 21.28 | 18.81 | 18.26 | 18.27 | 19.52 | 18.62 | 18.41 | 18.37 |
AS TS FS | 39.88 | 38.19 | 37.53 | 37.66 | 38.77 | 37.93 | 37.76 | 37.72 |
MS TS | 28.89 | 27.04 | 26.73 | 26.72 | 27.55 | 27.00 | 26.84 | 26.77 |
CH3 HS TS | 15.37 | 18.57 | 18.30 | 18.22 | 19.68 | 18.67 | 18.38 | 18.30 |
NH2 HS TS | 37.45 | 37.18 | 37.45 | 37.48 | 38.30 | 37.77 | 37.60 | 37.53 |
CH3 HA MIN′ | −50.54 | −48.76 | −48.61 | −48.68 | −47.64 | −48.20 | −48.44 | −48.51 |
CH3 HA MIN′′ | −53.07 | −51.01 | −50.93 | −50.99 | −49.89 | −50.49 | −50.74 | −50.80 |
NH2 HA MIN | −46.19 | −45.80 | −45.62 | −45.67 | −44.92 | −45.28 | −45.45 | −45.50 |
AS MIN W | −25.23 | −25.62 | −25.48 | −25.51 | −24.91 | −25.20 | −25.39 | −25.42 |
AS MIN FS | −26.20 | −26.62 | −26.50 | −26.50 | −25.85 | −26.19 | −26.37 | −26.40 |
MS MIN | 16.33 | 16.09 | 16.00 | 15.98 | 16.71 | 16.25 | 16.09 | 16.03 |
CH3 HS MIN | −23.20 | −18.36 | −17.71 | −17.79 | −16.95 | −17.39 | −17.64 | −17.69 |
NH2 HS MIN | 28.86 | 32.54 | 33.04 | 33.07 | 33.71 | 33.34 | 33.17 | 33.10 |
CH3NHF + H | 29.10 | 32.97 | 33.41 | 33.40 | 33.98 | 33.73 | 33.61 | 33.47 |
NH2F + CH3 | 18.45 | 18.26 | 18.01 | 17.89 | 18.54 | 18.13 | 17.96 | 17.91 |
CH2FNH2 + H | −23.03 | −18.03 | −17.44 | −17.54 | −16.77 | −17.07 | −17.28 | −17.42 |
CH3F + NH2 | −22.83 | −23.45 | −23.46 | −23.54 | −22.94 | −23.25 | −23.43 | −23.47 |
HF + CH3NH | −34.16 | −33.77 | −33.80 | −33.98 | −33.46 | −33.72 | −33.87 | −33.92 |
HF + CH2NH2 | −42.75 | −41.02 | −41.19 | −41.45 | −40.58 | −41.06 | −41.32 | −41.38 |
CH4 + NHF | 6.31 | 4.04 | 3.87 | 3.79 | 4.07 | 3.88 | 3.80 | 3.77 |
NH3 + CH2F | −29.71 | −29.77 | −29.89 | −30.05 | −29.16 | −29.65 | −29.92 | −29.96 |
Furthermore, this study reveals the basis-set effect of the results determined using the CCSD(T)-F12b method, comparing the aug-cc-pVnZ (n = D, T, Q) and the cc-pVnZ-F12 (n = D, T, Q, 5) basis sets. In Fig. 3 the relative energies, which are obtained using the above basis sets, are presented relative to the CCSD(T)-F12b/cc-pV5Z-F12 data. In the case of the aug-cc-pVDZ and aug-cc-pVTZ basis sets, the geometry optimizations are carried out, whereas in the other cases single-point computations are performed at the CCSD(T)-F12b/aug-cc-pVTZ geometries. With the increasing basis set, the average absolute deviations between the CCSD(T)-F12b/aug-cc-pVnZ and corresponding CCSD(T)-F12b/cc-pVnZ-F12 relative energies are decreasing, as 0.80 (n = D), 0.29 (n = T) and 0.15 (n = Q) kcal mol−1. If the results that are achieved using the cc-pV5Z-F12 basis set – used for benchmark values – are examined, as opposed to the results obtained with the mentioned other basis sets, we can establish that the root-mean-square deviation (RMSD) is less for the comparison of aug-cc-pVDZ and cc-pV5Z-F12 (RMSD = 0.37 kcal mol−1), than cc-pVDZ-F12 vs. cc-pV5Z-F12 (RMSD = 0.82 kcal mol−1). The inference is similar in the case of the aVTZ – V5Z-F12 and the VTZ-F12 – V5Z-F12 differences, where RMSDs are 0.09 and 0.26 kcal mol−1, respectively. The VQZ-F12 – V5Z-F12 difference is the least, with RMSD = 0.07 kcal mol−1. Thus, the CCSD(T)-F12b/cc-pV5Z-F12 relative energies are clearly basis-set-converged within 0.1 kcal mol−1.
The sum of the energy contributions of the core correlation and scalar relativistic effect is in the range of −0.02 to 0.52 kcal mol−1. These corrections play the most significant role in the case of the functional-group-substitution pathways, the finding of which is supported by the fact that Δcore + Δrel = 0.52 kcal mol−1 for the AS TS FS, whereas this value is only −0.01 kcal mol−1 for the CH3 HA TS′′. As this latter value shows, for the stationary points and products of the hydrogen-abstraction channels, these effects almost cancel each other.
The relativistic spin–orbit (SO) interactions in halogen atoms such as F may cause energy lowering in open-shell systems, and we have to take them into account in order to achieve chemical accuracy. The non-relativistic 2P ground state (non-SO) of the fluorine atom splits to the ground 2P3/2 state, with a lower energy by 1/3ε, and to the excited 2P1/2 state (SO3), with a higher energy by 2/3ε regarding the non-SO state, where ε is the SO splitting between the 2P3/2 and 2P1/2 states. While the F atom approaches the CH3NH2 molecule, the former fourfold degenerate state splits into two twofold degenerate states: the SO ground (SO1) and SO excited (SO2) states. Furthermore, three twofold degenerate states form as a result of the splitting of the non-SO ground state, resulting in a non-SO ground (non-SO1) and two non-SO excited states (non-SO2 and non-SO3). The potential energy curves of these six states, for the cases where the fluorine atom approaches the methylamine from seven different directions, are investigated and detailed in Subsection III.4. The SO-coupling-instigated energy decrease of the F atom is calculated as the difference between the asymptotic limits of the SO1 and non-SO1 potential energies. In general, this results in a SO correction of 0.37 kcal mol−1, whereas ΔSO is 0.34/0.36 kcal mol−1 for hydrogen-abstraction transition states and AS TS W, the effect of which will be further discussed below.
The energy contribution values, as mentioned above, do not exceed 1 kcal mol−1 (one exception), whereas the effect of the relative-energy shifting caused by the harmonic zero-point energy (ZPE) is more significant. For the reactant-like PREMIN and NH2 HA TS the ZPE correction is positive, but in all other cases ΔZPE is negative. The absolute correction for the products is larger than for the TS or MIN complexes, due to the decrease in the number of vibrational degrees of freedom in the product channels, or, for example, for the H-substitution pathways, due to the replacement of the high-frequency C–H/N–H stretching modes with lower-frequency C–F/N–F bonds. For example, the NH2 HS TS has a −3.79 kcal mol−1 ZPE correction, the NH2 HS MIN has −4.68 kcal mol−1, and this effect is −5.00 kcal mol−1 for the CH3NHF + H products.
Considering of all the energy corrections mentioned so far, we can compare the computed benchmark adiabatic relative energies (obtained using eqn (7)) with the experimental data, which are provided by the Active Thermochemical Tables (ATcT).42 The 0 K reaction enthalpy is determined by extracting the enthalpy of formation from the ATcT database for the reactants and products for the following three reaction pathways: hydrogen abstraction from the methyl group (CH3 HA) and from the amino group (NH2 HA) and for amino substitution (AS), where ΔH0 = −43.98 ± 0.11, −37.22 ± 0.12 and −26.10 ± 0.08 kcal mol−1, respectively. By comparison, the present benchmark ab initio ΔH0 values are −44.12, −37.24 and −26.48 kcal mol−1 in order. In the case of NH2 HA, the agreement between the experimental and theoretical data is outstanding, where the deviation is only 0.02 kcal mol−1. Although for the other two pathways the differences are not within the ATcT uncertainty determined via Gaussian propagation law, they are nevertheless well below the 1 kcal mol−1 chemical accuracy.
To investigate the reactive SO1 state corresponding to the different orientations of separation, we illustrate the potential curves of this state in Fig. 4B. The van der Waals wells are shallow with depths of 0.2–0.5 kcal mol−1 in the region of 3.0–3.5 Å, when fluorine approaches the C or N atom from the hydrogen (C2, N2), the C atom along the line of the C–N bond (C1) or the C–N bond perpendicularly (CN2). The wells are deeper for the cases where F approaches methylamine from the more polar amino group in line with (N1) or perpendicularly to (CN1) the C–N bond, and are the deepest (−7.0 kcal mol−1 at 2 Å distance) when the angle of the approach is equal to the C–N–F angle obtained in PREMIN (N3). Here it is important to note that MRCI+Q seriously underestimates the well depth of PREMIN as the benchmark value is 20.87 kcal mol−1 (see Fig. 1) and the geometry effect corresponding to fixed vs. relaxed CH3NH2 is only 0.1 kcal mol−1. These findings indicate that dynamical electron correlation, even beyond single and double excitations, is important for describing the F⋯CH3NH2 interaction. To further support this conclusion, we report that the PREMIN complex is unbound at the ROHF level, and CCSD-F12b underestimates the CCSD(T)-F12b well depth by 3.7 kcal mol−1. These extremely large correlation effects are not unprecedented as the barrier height of the F + CH4 reaction is 9.41, 1.80, and 0.55 kcal mol−1 at the ROHF, CCSD, and CCSD(T) levels of theory, respectively.10
The two additional kinetically hindered product channels are also investigated in the title reaction, where two bonds are required to break and form, leading to NH3 + CH2F and CH4 + NHF. The products of the former channel have a ΔE(ΔH0) = −29.65(−32.77) kcal mol−1 relative energy and those of the latter have a ΔE(ΔH0) = 3.87(0.03) kcal mol−1, whereas the corresponding values for the Cl + CH3NH2 system are −3.70(−8.20) and 14.24(9.82) kcal mol−1, respectively.22 In both reactions, the exothermic NH3 formation is thermodynamically competitive with H-abstractions; however, the former may have a much higher barrier due to the two bond cleavages required to form the NH3 + CH2F/CH2Cl products.
The calculated SO interactions, which lower the asymptote of the reactants by 0.84 kcal mol−1 and 0.37 kcal mol−1 for the chlorine and fluorine reactions, respectively, show good agreement with the measured SO splittings (εexpt)43 of these atoms, as εexpt/3 = 0.84 (Cl) kcal mol−1 and 0.39 (F) kcal mol−1. Considering the different separation orientations of the reactants, it can be seen for both systems that the deepest van der Waals wells of the potential energy curves occur when the halogen atom approaches the amino group of methylamine with the angle of C–N–Cl/F obtained at the PREMIN geometry. The depth is shallower if the F/Cl atom attacks the molecule at the side of the hydrogens or at the methyl group in line with the C–N bond. The distance dependence of the SO – non-SO ground-state energy difference for the different approaching orientations is quite similar for the two systems.
The above-indicated subchemically accurate benchmark energy values are obtained by taking into account further energy contributions besides the CCSD(T)-F12b/cc-pV5Z-F12 single-point energies obtained at the CCSD(T)-F12b/aug-cc-pVTZ geometries. As we have already seen in the case of the reaction between chlorine and methylamine,22 the zero-point energy correction is the most significant among the energy contributions determined by us. By considering the post-(T) correlation (which mainly affects the energy of the TS structures and often by more than 1 kcal mol−1), the core electron correlation and scalar relativistic effects (which are usually negligible to reach chemical accuracy but are often needed if subchemical accuracy is desired), and the SO correction of about 0.37 kcal mol−1, we are able to achieve excellent agreement with the experimental results for the three product channels, where data are available in the ATcT database.
This study provides guidance for the development of a global analytical ab initio potential energy surface required for the first-principles investigation of reaction dynamics, thereby enabling a comparison with the results obtained via experiments, and gaining deeper atomic insight into the mechanisms of the F + CH3NH2 reaction.
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