Kinetics and mechanism for the CH2O + NO2 reaction: A computational study

The reactants, products, and transition states of the CH₂O + NO₂ reaction on the ground electronic potential energy surface have been searched at both B3LYP/6?311+G(d,p) and MPW1PW91/6?311+G(3df,2p) levels of theory. The forward and reverse barriers are further improved by a modified Gaussian‐2 method. The theoretical rate constants for the two most favorable reaction channels 1 and 2 producing CHO + cis‐HONO and CHO + HNO₂, respectively, have been calculated over the temperature range from 200 to 3000 K using the conventional and variational transition‐state theory with quantum‐mechanical tunneling corrections. The former product channel was found to be dominant below 1500 K, above which the latter becomes competitive. The predicted total rate constants for these two product channels can be presented by k_t (T) = 8.35 × 10^?11 T^6.68 exp(?4182/T) cm³/(mol s). The predicted values, which include the significant effect of small curvature tunneling corrections, are in quantitative agreement with the available experimental data throughout the temperature range studied (390–1650 K). © 2003 Wiley Periodicals, Inc. Int J Chem Kinet 35: 184–190, 2003     

Understanding the Initial Decomposition Pathways of the n‐Alkane/Nitroalkane Binary Mixture

Addition of nitroalkanes into n‐alkanes can lower the activation barriers of free‐radical production and accelerate the decomposition of n‐alkanes at relatively low temperatures. Four initial decomposition mechanisms of the n‐butane/nitroethane binary mixture were proposed for the promoting effect and considered theoretically at the B3LYP, BB1K, BMK, MPW1K, and M06‐2X levels with MG3S basis set. Energetics above was compared to high‐level CBS‐QB3 and G4 calculations. Calculated results confirm the feasibility of the four initial decomposition pathways: (I) the C? NO₂ bond rupture of nitroethane to produce ethyl and ·NO₂, (II) HONO elimination from nitroethane followed by decomposition to ·OH and ·NO, (III) rearrangement of nitroethane to ethyl nitrite which further dissociates into CH₃CH₂O· and ·NO, and (IV) direct hydrogen‐abstraction of nitroethane with n‐butane.     

The Spin Gap Transition around 180 K Observed in a New Molecular Magnet based on the [Ni(mnt)2]– Monoanion and Substituted Benzylpyridinium

A new molecular magnet, [NO₂BrBzPyCH₃][Ni(mnt)₂] ( 1 ) ([NO₂BrBzPyCH₃]⁺ = 1‐(2′‐bromo‐4′‐nitrobenzyl)‐2‐methylpyridinium, and mnt^2– = maleonitriledithiolate), has been prepared and characterized by single crystal X‐ray diffraction and magnetic measurements. The Ni(III) ions of 1 form a quasi‐one‐dimensional alternating zig‐zag magnetic chain within a Ni(mnt)₂^– column by intermolecular Ni···S, Ni···Ni or π···π interactions, and the [NO₂BrBzPyCH₃]⁺ cations stack into a column via weak Br···O interaction, p···π stacking interactions and C‐H···O hydrogen bonds between the cations. Magnetic susceptibility measurements in the temperature range 1.8‐300 K show that 1 exhibits a spin‐gap transition around 180 K, and an antiferromagnetic interaction in the high‐temperature phase (HT) and spin gap behavior in the low‐temperature phase (LT). The transition for 1 is a second‐order phase transition as determined by DSC analyses.     

DFT studies on the multi‐channel reaction of CH3S+NO2

The mechanisms for the reaction of CH₃S with NO₂ are investigated at the QCISD(T)/6‐311++G(d,p)//B3LYP/6‐311++G(d,p) on both single and triple potential energy surfaces (PESs). The geometries, vibrational frequencies, and zero‐point energy (ZPE) correction of all stationary points involved in the title reaction are calculated at the B3LYP/6‐311++G(d,p) level. More accurate energies are obtained at the QCISD(T)/6‐311++G(d,p). The results show that 5 intermediates and 14 transition states are found. The reaction is more predominant on the single PES, while it is negligible on the triple PES. Without any barrier height for the whole process, the main channel of the reaction is to form CH₃SONO and then dissociate to CH₃SO+NO. © 2006 Wiley Periodicals, Inc. Int J Quantum Chem, 2007     

Five solvates of a multicomponent pharmaceutical salt formed by berberine and diclofenac

A multicomponent pharmaceutical salt formed by the isoquinoline alkaloid berberine (5,6‐dihydro‐9,10‐dimethoxybenzo[g]‐1,3‐benzodioxolo[5,6‐a]quinolizinium, BBR) and the nonsteroidal anti‐inflammatory drug diclofenac {2‐[2‐(2,6‐dichloroanilino)phenyl]acetic acid, DIC} was discovered. Five solvates of the pharmaceutical salt form were obtained by solid‐form screening. These five multicomponent solvates are the dihydrate (BBR–DIC·2H₂O or C₂₀H₁₈NO₄⁺·C₁₄H₁₀Cl₂NO₂^?·2H₂O), the dichloromethane hemisolvate dihydrate (BBR–DIC·0.5CH₂Cl₂·2H₂O or C₂₀H₁₈NO₄⁺·C₁₄H₁₀Cl₂NO₂^?·0.5CH₂Cl₂·2H₂O), the ethanol monosolvate (BBR–DIC·C₂H₅OH or C₂₀H₁₈NO₄⁺·C₁₄H₁₀Cl₂NO₂^?·C₂H₅OH), the methanol monosolvate (BBR–DIC·CH₃OH or C₂₀H₁₈NO₄⁺·C₁₄H₁₀Cl₂NO₂^?·CH₃OH) and the methanol disolvate (BBR–DIC·2CH₃OH or C₂₀H₁₈NO₄⁺·C₁₄H₁₀Cl₂NO₂^?·2CH₃OH), and their crystal structures were determined. All five solvates of BBR–DIC (1:1 molar ratio) were crystallized from different organic solvents. Solvent molecules in a pharmaceutical salt are essential components for the formation of crystalline structures and stabilization of the crystal lattices. These solvates have strong intermolecular O…H hydrogen bonds between the DIC anions and solvent molecules. The intermolecular hydrogen‐bond interactions were visualized by two‐dimensional fingerprint plots. All the multicomponent solvates contained intramolecular N—H…O hydrogen bonds. Various π–π interactions dominate the packing structures of the solvates.     

Theoretical study of the mechanism of CH2CO + CN reaction

The potential energy surface information of the CH₂CO + CN reaction is obtained at the B3LYP/6‐311+G(d,p) level. To gain further mechanistic knowledge, higher‐level single‐point calculations for the stationary points are performed at the QCISD(T)/6‐311++G(d,p) level. The CH₂CO + CN reaction proceeds through four possible mechanisms: direct hydrogen abstraction, olefinic carbon addition–elimination, carbonyl carbon addition–elimination, and side oxygen addition–elimination. Our calculations demonstrate that R→IM1→TS3→P3: CH₂CN + CO is the energetically favorable channel; however, channel R→IM2→TS4→P4: CH₂NC + CO is considerably competitive, especially as the temperature increases (R, IM, TS, and P represent reactant, intermediate, transition state, and product, respectively). The present study may be helpful in probing the mechanism of the CH₂CO + CN reaction. © 2005 Wiley Periodicals, Inc. Int J Quantum Chem, 2006     

Ab initio and DFT Study of the Structural Properties and Thermochemistry of CH3S(O)2OONO2 Atmospheric Molecule and CH3S(O)2OO· Radical

The conformational properties of methanesulfonyl peroxynitrate, CH₃S(O)₂OONO₂ (MSPN), and its radical decomposition products CH₃S(O)₂OO· and CH₃S(O)₂O· were studied by ab initio and density functional methods. The dihedral angle around the S–O and the O–O single bond are calculated to be ?70.5° and ?97.8° (B3LYP/6‐311++G(3df,3pd)), respectively. The principal unimolecular dissociation pathways for MSPN were studied using complete basis set (CBS) methods. The reaction enthalpies for the channels CH₃S(O)₂OONO₂→ CH₃S(O)₂OO·+NO₂ and CH₃S(O)₂OONO₂→CH₃S(O)₂O·+NO₃ were computed to be 111.0 and 140.9 kJ/mol, respectively. The enthalpies of formation at 298 K for MSPN and CH₃S(O)₂OO radical were predicted to be ?358.2 and ?281.3 kJ/mol, respectively.     

Synthesis,Characterization, and Luminescent Properties of Silver(I) Complexes based on Diphosphine Ligands and 6,7‐Dicyanodipyridoquinoxaline

Five mono‐nuclear silver (I) complexes with 6,7‐dicyanodipyridoquinoxaline ligand, namely {[Ag(DPEphos)(dicnq)]NO₃}₂ · CH₃OH ( 1 ), [Ag(DPEphos)(dicnq)]BF₄ · CH₃OH ( 2 ), [Ag(XANTphos)(dicnq)]CF₃SO₃ ( 3 ), {[Ag(XANTphos)(dicnq)]NO₃}₂ ( 4 ), and [Ag(XANTphos)(dicnq)]ClO₄ · CH₂Cl₂ ( 5 ) {DPEphos = bis[2‐(diphenylphosphanyl)phenyl]ether, dicnq = 6,7‐dicyanodipyridoquinoxaline, XANTphos = 9,9‐dimethyl‐4,5‐bis(diphenylphosphanyl)xanthene} were characterized by X‐ray diffraction, IR, ¹H NMR, ³¹P NMR, fluorescence spectra, and terahertz time‐domain spectra (THz‐TDS). In the five complexes the Ag^I, which is coordinated by two kinds of chelating ligands, adopts four‐coordinate modes to generate mono‐nuclear structures. The C–H ··· π interactions lead to formation of a 1D infinite chain for complexes 2 and 3 . The crystal packing of complexes 1 and 5 reveal that they form 3D supermolecular network by several pairs of C–H ··· π interactions. The emissions of these complexes are attributed to ligands‐centered [π–π*] transition based on both of the P‐donor and N‐donor ligands.     

Theoretical kinetic investigation of thermal decomposition of nitropropane

The thermal decomposition of nitropropane (CH₃CH₂CH₂NO₂) has been investigated at the CBS-QB3 level of theory. The pyrolysis of CH₃CH₂CH₂NO₂ mainly includes the simple bond ruptures mechanism, hydrogen abstraction processes, isomerization and secondary reactions. As a result, for the simple bond ruptures mechanism, the formation of \({\text{CH}}_{3} {\text{CH}}_{2} {\text{CH}}_{2}^{\cdot} +\,^{\cdot}{\text{NO}}_{2}\) products is dominant with the energy barrier of 49.77 kcal mol^?1. The process of H atom on the β–CH₂ abstracted by one O atom of NO₂ moiety in CH₃CH₂CH₂NO₂(CH₃CH₂CH₂NO₂ → CH₃CH=CH₂ + HONO) needs to overcome lower energy barrier than that of the rate-determining step (one of H atom on the α-CH₂ and γ-CH₃ abstracted of reaction) of the other hydrogen abstraction reactions. Therefore, we predict that the corresponding alkenes and HONO are the main products in the hydrogen abstraction reaction of nitroparaffin. Besides, the channel of the CH₃CH₂CHO + HNO formations (CH₃CH₂C(α)H₂NO₂ → CH₃CH₂C(α)H₂ONO → CH₃CH₂CHO + HNO), occurring through the H atom of C(α) abstracted by the N atom of NO₂ moiety after the isomerization reaction from CH₃CH₂CH₂NO₂ to CH₃CH₂CH₂ONO, is favorable in the isomerization secondary reactions. Rate constants and branching ratios are estimated by means of the conventional transition state theory with zero curvature tunneling over the temperature range of 400–1500 K. The calculation shows that the overall rate constant in the temperature of 400–1500 K is mainly dependent on the competitive channels of formations of CH₃CH=CH₂ + HONO and \({\text{CH}}_{3} {\text{CH}}_{2} {\text{CH}}_{2}^{\cdot} +\,^{\cdot}{\text{NO}}_{2}\) The three-parameter expression for the total rate constant is fitted to be k _total = 1.74 × 10^?13 T ^8.20exp(17038.7/T) (s^?1) between 400 and 1500 K.     

Theoretical Study of the Rate Coefficients for CH3NHNH2 + NO2 and Related Reactions

The kinetics and mechanisms of H atom abstraction reactions from CH₃NHNH₂ by NO₂ (R1) and related reactions have been investigated theoretically by using ωB97X‐D and CCSD(T)‐F12 quantum chemical calculations and the steady‐state unimolecular master equation analysis based on Rice–Ramsperger–Kassel–Marcus (RRKM) theory. For reaction (R1), both dissociation and isomerization steps between intermediate complexes were found to be important for the distribution of the dissociated bimolecular products. The dominant products of (R1) were found to be cis‐CH₃NHNH and HONO at lower temperature. The branching ratios for CH₃NNH₂ formation paths increased with increasing temperature. On the same reaction potential energy surface, six reactions including isomerization reactions between CH₃NNH₂ and cis‐/trans‐CH₃NHNH catalyzed by HONO were suggested to compete with the reverse reaction of (R1). The temperature‐ and pressure‐dependent rate expressions are proposed for kinetic modeling.     

Isomerizations of CH3SO2: Quantum Chemistry and Topological Study

The structures, stabilities and the isomerization reactions of CH₃SO₂ isomers in a doublet electronic state have been studied at B3LYP/6‐311+ +G (d,p), MP2/6‐311++G (d,p) and CCSD(T)/6‐311++G (d,p) levels. The three different levels of calculation give the similar results: thirteen minimum isomers were located and they were connected by eleven transition states. Among the thirteen isomers, cis‐CH₃OSO, trans‐CH₃OSO and CH₃SO₂ are the most stable species, and they should be detected easily in experiment. This is well consistent with the experimental result. These isomers could isomerize to each other by chemical bond vibration, chemical bond rotation and atom migration. The non‐planar ring structure transition state (STS), which was found in this paper, extended the concept of ring STS to the non‐planar systems.     

DFT investigation of the mechanism of CH2CO + O(3P) reaction

A detailed theoretical survey of the potential energy surface (PES) for the CH₂CO + O(³P) reaction is carried out at the QCISD(T)/6‐311+G(3df,2p)//B3LYP/6‐311+G(d,p) level. The geometries, vibrational frequencies, and energies of all stationary points involved in the reaction are calculated at the B3LYP/6‐311+G(d,p) level. More accurate energy information is provided by single‐point calculations at the QCISD(T)/6‐311+G(3df,2p) level. Relationships of the reactants, transition states, intermediates, and products are confirmed by the intrinsic reaction coordinate (IRC) calculations. The results suggest that P1(CH₂+CO₂) is the most important product. This study presents highlights of the mechanism of the title reaction. © 2005 Wiley Periodicals, Inc. Int J Quantum Chem, 2005     

DFT studies on the mechanism of the reaction of C2H5S with NO2

The mechanisms for the reaction of C₂H₅S with NO₂ are investigated at the QCISD(T)/6‐311++G(d, p)//B3LYP/6‐311++G(d, p) level on both single and triple potential energy surfaces. The geometries, vibrational frequencies and zero‐point energy (ZPE) corrections of all stationary points involved in the title reaction are calculated at the B3LYP/6‐311++G(d, p) level. The results show that the reaction is more predominant on the single potential energy surface, while it is negligible on the triple potential energy surface. Without barrier height in the whole process, the major channel is R → C₂H₅SONO (IM1 and IM2) → P1 (C₂H₅SO+NO). With much heat released in the formation of C₂H₅SNO₂ (IM3) and the transition state involved in the subsequent step more stable than reactants, P4 (CH₃CHS + t‐HONO) is subdominant product energetically. © 2007 Wiley Periodicals, Inc. Int J Quantum Chem, 2007     

Ab initio chemical kinetics for the NH2 + HNOx reactions,part II: Kinetics and mechanism for NH2 + HONO

The kinetics and mechanism for the reaction of NH₂ with HONO have been investigated by ab initio calculations with rate constant prediction. The potential energy surface of this reaction has been computed by single‐point calculations at the CCSD(T)/6‐311+G(3df, 2p) level based on geometries optimized at the CCSD/6‐311++G(d, p) level. The reaction producing the primary products, NH₃ + NO₂, takes place via precomplexes, H₂N???c‐HONO or H₂N???t‐HONO with binding energies, 5.0 or 5.9 kcal/mol, respectively. The rate constants for the major reaction channels in the temperature range of 300–3000 K are predicted by variational transition state theory or Rice–Ramsperger–Kassel–Marcus theory depending on the mechanism involved. The total rate constant can be represented by k_total = 1.69 × 10^?20 × T^2.34 exp(1612/T) cm³ molecule^?1 s^?1 at T = 300–650 K and 8.04 × 10^?22 × T^3.36 exp(2303/T) cm³ molecule^?1 s^?1 at T = 650–3000 K. The branching ratios of the major channels are predicted: k₁ + k₃ producing NH₃ + NO₂ accounts for 1.00–0.98 in the temperature range 300–3000 K and k₂ producing OH + H₂NNO accounts for 0.02 at T > 2500 K. The predicted rate constant for the reverse reaction, NH₃ + NO₂ → NH₂ + HONO represented by 8.00 × 10^?26 × T^4.25 exp(?11,560/T) cm³ molecule^?1 s^?1, is in good agreement with the experimental data. © 2009 Wiley Periodicals, Inc. Int J Chem Kinet 41: 678–688, 2009     

Computational study on the kinetics and mechanisms for the reactions of HCO with HONO and HNOH

The kinetics and mechanisms of the HCO reactions with HONO and HNOH have been studied at the G2M level of theory based on the geometric parameters optimized at BH&HLYP/6‐311G(d,p). The rate constants in the temperature range 200–3000 K at different pressures have been predicted by microcanonical RRKM and/or variational transition state theory calculations with Eckart tunneling corrections. For the HCO + HONO reaction, hydrogen abstraction from trans‐HONO and cis‐HONO by HCO produces H₂CO + NO₂, with the latter being dominant. Two other channels involving cis‐HONO by the association/decomposition mechanism via the HC(O)N(O)OH intermediate, which could fragment to give H₂O + CO + NO at high temperatures, were also found to be important. For the HCO + HNOH reaction, three reaction channels were identified: one association reaction giving a stable intermediate, HC(O)N(H)OH (LM2), and two hydrogen abstraction channels producing H₂CO and H₂NOH. The dominant products were predicted to be the formation of LM2 at low temperatures and H₂NOH + CO at middle and high temperatures. © 2004 Wiley Periodicals, Inc. Int J Chem Kinet 36: 178–187 2004     

Differences and similarities among hypoxanthinium nitrate hydrate structures

Crystals of hypoxanthinium (6‐oxo‐1H,7H‐purin‐9‐ium) nitrate hydrates were investigated by means of X‐ray diffraction at different temperatures. The data for hypoxanthinium nitrate monohydrate (C₅H₅N₄O⁺·NO₃^?·H₂O, Hx1 ) were collected at 20, 105 and 285 K. The room‐temperature phase was reported previously [Schmalle et al. (1990). Acta Cryst. C 46 , 340–342] and the low‐temperature phase has not been investigated yet. The structure underwent a phase transition, which resulted in a change of space group from Pmnb to P2₁/n at lower temperature and subsequently in nonmerohedral twinning. The structure of hypoxanthinium dinitrate trihydrate (H₃O⁺·C₅H₅N₄O⁺·2NO₃^?·2H₂O, Hx2 ) was determined at 20 and 100 K, and also has not been reported previously. The Hx2 structure consists of two types of layers: the `hypoxanthinium nitrate monohydrate' layers (HX) observed in Hx1 and layers of Zundel complex H₃O⁺·H₂O interacting with nitrate anions (OX). The crystal can be considered as a solid solution of two salts, i.e. hypoxanthinium nitrate monohydrate, C₅H₅N₄O⁺·NO₃^?·H₂O, and oxonium nitrate monohydrate, H₃O⁺(H₂O)·NO₃^?.     

新的杯[4]芳烃衍生物通过缔合乙腈进行自插入的X-射线衍射和密度泛函理论研究

合成和表征了一个新的杯[4]芳烃衍生物，11,23-二羟亚胺甲基-25,27-二羟基-26,28-二丙氧基杯[4]芳烃 (B)及其与乙腈生成的组成为B·2CH₃CN的化合物。¹H NMR显示，在B·2CH₃CN中B采取锥型构象，X-射线衍射分析确证在溶液中所发现的构象。在晶格网络中存在着B·2CH₃CN以二聚体形式的自插入现象。在B3LYP/6-311G(d)水平上计算了该自插入二聚体中的非共价相互作用能，并对基集叠加误差进行了校正。在二聚体中的B·2CH₃CN，一个CH₃CN通过与羟亚胺基形成氢键使之稳定，结合能为–5.02 kJ·mol^-1，另一个CH₃CN则通过与另一个羟亚胺基形成氢键以及与另一B·2CH₃CN中B苯环空腔间的C–H···π相互作用使之稳定，结合能分别为–14.23 kJ·mol^-1和–3.77 kJ·mol^-1。自插入的驱动能为–7.54 kJ·mol^-1。     

Theoretical study on the reaction mechanism of CH2SH + NO2

The mechanisms of CH₂SH with NO₂ reaction were investigated on the singlet and triplet potential energy surfaces (PES) at the BMC-CCSD//B3LYP/6-311 + G(d,p) level. The result shows that the title reaction is more favourable on the singlet PES thermodynamically, and it is less competitive on the triplet PES. On the singlet PES, the initial addition of CH₂SH with NO₂ leads to HSCH₂NO₂ (IM2) without any transition state, followed by a concerted step involving C–N fission and shift of H atom from S to O giving out CH₂S + trans-HONO, which is the major products of the title reaction. With higher barrier height, the minor products are CH₂S + HNO₂, formed by a similar concerted step from the initial adduct HSCH₂ONO (IM1). The direct abstraction route of H atom in SH group abstracted by O atom might be of some importance. It starts from the addition of the reactants to form a weak interaction molecular complex (MC3), subsequently, surmounts a low barrier height leading to another complex (MC2), which gives out CH₂S + trans-HONO finally. Other direct hydrogen abstraction channels could be negligible with higher barrier heights and less stable products.     

Influence of Isotope Substitution on Lattice and Spin‐Peierls‐Type Transition Features in One‐Dimensional Nickel Bis‐dithiolene Spin Systems

Four new 1D spin‐Peierls‐type compounds, [D₅]1‐(4′‐R‐benzyl)pyridinium bis(maleonitriledithiolato)nickelate ([D₅]R‐Py; R=F, I, CH₃, and NO₂), were synthesized and characterized structurally and magnetically. These 1D compounds are isostructural with the corresponding non‐deuterated compounds, 1‐(4′‐R‐benzyl)pyridinium bis(maleonitriledithiolato)nickelate (R‐Py; R=F, I, CH₃, and NO₂). Compounds [D₅]R‐Py and R‐Py (R=F, I, CH₃, and NO₂) crystallize in the monoclinic space group P2₁/c with uniform stacks of anions and cations in the high‐temperature phase and triclinic space group P$\bar 1$ with dimerized stacks of anions and cations in the low‐temperature phase. Similar to the non‐deuterated R‐Py compounds, a spin‐Peierls‐type transition occurs at a critical temperature for each [D₅]R‐Py compound; the magnetic character of the 1D S=1/2 ferromagnetic chain for [D₅]F‐Py and the 1D S=1/2 Heisenberg antiferromagnetic chain for others appear above the transition temperature. Spin‐gap magnetic behavior was observed for all of these compounds below the transition temperature. In comparison to the corresponding R‐Py compound, the cell volume is almost unchanged for [D₅]F‐Py and shows slight expansion for [D₅]R‐Py (R=I, CH₃, and NO₂) as well as an increase in the spin‐Peierls‐type transition temperature for all of these 1D compounds in the order of F>I≈CH₃≈NO₂. The large isotopic effect of nonmagnetic countercations on the spin‐Peierls‐type transition critical temperature, T_C, can be attributed to the change in ω₀ with isotope substitution.     

A phase transition from monoclinic C2 with Z′ = 1 to triclinic P1 with Z′ = 4 for the quasiracemate l‐2‐aminobutyric acid–d‐methionine (1/1)

Racemates of hydrophobic amino acids with linear side chains are known to undergo a unique series of solid‐state phase transitions that involve sliding of molecular bilayers upon heating or cooling. Recently, this behaviour was shown to extend also to quasiracemates of two different amino acids with opposite handedness [Görbitz & Karen (2015). J. Phys. Chem. B, 119 , 4975–4984]. Previous investigations are here extended to an l ‐2‐aminobutyric acid–d ‐methionine (1/1) co‐crystal, C₄H₉NO₂·C₅H₁₁NO₂S. The significant difference in size between the –CH₂CH₃ and –CH₂CH₂SCH₃ side chains leads to extensive disorder at room temperature, which is essentially resolved after a phase transition at 229 K to an unprecedented triclinic form where all four d ‐methionine molecules in the asymmetric unit have different side‐chain conformations and all three side‐chain rotamers are used for the four partner l ‐2‐aminobutyric acid molecules.