| The intermolecular weak interactions involving fluorine and/or chlorine atoms play an extremely important role in the fields of atmospheric chemistry,supramolecular chemistry,biochemistry,chiral recognition,and drug design.Therefore,it is very important to understand the characteristics of structures and energies of such weak interactions as well as their bonding rules.Fluorine-/chlorine-containing alkane molecular clusters can be used as a good prototype system to study the properties of such weak interactions at the molecular level.The length of the carbon chain of the haloalkanes as well as the number and type of halogen atoms in haloalkanes will affect the geometric structures,conformational equilibrium,the bonding modes of intermolecular weak interactions and the internal dynamics of molecular clusters.High resolution and high sensitivity Fourier transform microwave spectroscopy as a powerful gas phase spectroscopic technique,which are free from the crystal or solvation effects,can directly provide information on accurate structures,conformational equilibrium,energies and internal dynamics of small molecules and their clusters.The hyperfine structures caused by the nitrogen or chlorine nuclear quadrupole coupling effect can also be determined and resolved.Herein,the rotational spectra of the hetero-and homo-clusters of several alkanes containing fluorine or chlorine atoms have been investigated by using Fourier transform microwave spectroscopy in combination with molecular dynamics conformational search and quantum chemistry theoretical calculation methods in this paper.We studied the cooperativity of weak hydrogen bonds involving fluorine or chlorine atoms in the homo-and hetero-dimers and trimers,the structural and energetic characteristics of fluorine-/chlorine-containing chalcogen bonds,as well as the bonding rules of such interactions.Simultaneously,we also explored the large amplitude motions concerning the water molecule as well as methyl and trifluoromethyl groups.The main content of this paper includes the following five parts:In order to study the effect of the length of the carbon chain of the halogenated alkanes on the total interaction energy of its monohydrate,the rotational spectrum of the CH2FCH2CH2F···H2O complex in the first part was investigated by using high-resolution Fourier transform microwave spectroscopy and quantum chemistry calculations.Only the most stable conformer of the CH2FCH2CH2F···H2O complex has been observed experimentally in the supersonic expansion,and the molecular structure of this observed conformer was accurately determined.Water molecule simultaneously acting as the weak hydrogen bond donor and acceptor interacts with 1,3-difluoropropane molecule through one O-H···F weak hydrogen bond and two C-H···O weak hydrogen bonds.The length and strength of the H···F weak hydrogen bond have been determined to be 2.141(5)·and-18.83 k J mol-1,respectively.The presence of two C-H···O secondary interactions hinders the large amplitude motion of water molecule around its symmetry axis.Compared to a series of the 1:1 molecular complexes of water and n-alkanes containing fluorine and/or chlorine atoms,symmetry-adapted perturbation theory(SAPT)analysis indicates that the total interaction energies of such monohydrates increases with the increase of carbon chain length.To explore the influence of the number of the halogen atoms on the number of structures and total interaction energies of the halogenated n-alkanes,the rotational spectra of the 1:1 molecular clusters formed by CH2F2 with CH3CHF2 and CF3CH2F,respectively,were characterized using Fourier transform microwave spectroscopy in the second part.The three most stable conformers of both CH2F2···CH3CHF2 and CH2F2···CF3CH2F complexes were observed in microwave spectroscopic experiments.No tunneling splittings of the rotational transition lines caused by the internal rotation of methyl or trifluoromethyl groups in both CH2F2···CH3CHF2 and CH2F2···CF3CH2F complexes was observed.Quantum theory of atoms in molecules(QTAIM)and non-covalent interaction(NCI)analyses indicate that each observed conformer in both complexes is stabilized through three intermolecular C-H···F-C weak hydrogen bonds.The lengths and energies of the H···F weak hydrogen bonds range from 2.487·to 2.891·and range from-4.4 k J mol-1to-8.8 k J mol-1,respectively.The research results also show that the total interaction energies of the clusters decrease with the increase of the number of fluorine atoms.In order to investigate the cooperativity between the same type of weak hydrogen bonds,the CREST conformational search and quantum theoretical calculation methods were first used to explore the intricate conformational space of(CH3CHF2)3(E3),CH2F2···(CH3CHF2)2(ME2)and(CH2F2)2···CH3CHF2(M2E)clusters in the third part.In total,more than 100 stable conformers for each cluster were identified to be true minima.Subsequently,the first two most stable conformers of each cluster were observed by microwave spectroscopy.The three subunits in all the three clusters are held together by a network of 9 or 10 intermolecular C-H···F-C weak hydrogen bonds.The lengths of the C-H and C-F covalent bond involved in the formation of the C-H···F-C weak hydrogen bond in the M2,ME and E2 dimers as well as M3,M2E,ME2 and E3 trimers were calculated at the B97D/aug-cc-p VTZ level of theory,the results show that the average length of C-H(C-F)covalent bond decreases(increases)from dimer to trimer.Besides,the average length of C-H(C-F)covalent bond in both dimers and trimers further decreases(increases)as the number of the E subunit increases.The theoretical results also reveal that the total interaction energies of both dimers and trimers decrease as the number of the E subunit increases.The results demonstrate that the C-H···F-C weak hydrogen bonds have a positive synergistic effect in both homotrimers and heterotrimers.In order to characterize the nature of the N-H···F weak hydrogen bond,the rotational spectrum of the CH2F2···HCONH2 dimer was studied by using microwave spectroscopy with the help of theoretical calculations in the fourth part.Experimentally,only the most stable conformer of the dimer was detected in the supersonic expansion.The CH2F2moiety in this observed conformer interacts with HCONH2 through one N-H···F weak hydrogen bond and two C-H···O weak hydrogen bonds.The length and angle of the N-H···F weak hydrogen bond have been determined to be 2.140(14)·and 150.7(8)°,respectively.The results of the QTAM analysis show that the strength of the N-H···F weak hydrogen bond is-13.2 k J mol-1.The SAPT analysis result indicates that the electrostatic and dispersive components are the main factors for the stability of the CH2F2···HCONH2 dimer.The dissociation energy of the CH2F2···HCONH2 dimer has been estimated to be 15.9 k J mol-1 using a pseudo-diatomic approximation model.In the fifth part,high-resolution Fourier transform microwave spectroscopy and quantum chemical theoretical calculations were used to investigate the binary complexes of 2,2,4,4-tetrafluoro-1,3-dithietane(C2F4S2)with CH2F2 and CH3Cl,respectively,for the purpose of understanding the structural and energetic characteristics of the S···X(X=F,Cl,Br,I)chalcogen bonds as well as the bonding rules of such interactions.Only the most stable conformer of each cluster was observed experimentally.For the C2F4S2···CH3Cl cluster,the internal rotational potential barriers of the methyl groups of the 35Cl and 37Cl isotopologues have been determined to be 70.1097(4)cm-1and 70.144(1)cm-1,respectively.QTAM and NCI analyses reveal that both clusters are stabilized by one S···F/Cl chalcogen bond,two F···F/Cl contacts and two C-H···F-C weak hydrogen bonds.The lengths of the S···F and S···Cl chalcogen bond have been determined to be2.9759(6)·and 3.384(3)·,respectively,and the corresponding interaction energies are-10.4 k J mol-1and-7.0 k J mol-1,respectively.The research results indicate that the strengths of the S···X chalcogen bonds decrease as the size of the halogen atom increases,and the strengths of such chalcogen bonds are significantly weaker than those of the S···Y(Y=N,O,S)chalcogen bonds.Energy decomposition analysis based on the SAPT approach shows that the electrostatic and dispersion interactions play a key role in the stabilization of both C2F4S2···CH2F2 and C2F4S2···CH3Cl clusters. |
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