Intermolecular Interactions between Formaldehyde and Dimethyl Ether and between Formaldehyde and Dimethyl Sulfide in the Complex, Investigated by Fourier Transform Microwave Spectroscopy and Ab Initio Calculations Yoshiyuki Kawashima, Yoshio Tatamitani, and Yoshihiro Osamura Department of Applied Chemistry, Faculty of Engineering, Kanagawa Institute of Technology, Atsugi, Kanagawa , Japan Eizi Hirota The Graduate University for Advanced Studies, Hayama, Kanagawa , Japan

Intermolecular Interactions (1) Hydrogen bondstrong Dipole-dipole interaction Van der Waals force weak Y. Tatamitani et. al. J. Am. Chem. Soc. 124 (2002) 2739 (DME) 2, DME-DFE, DME-TFE, and DMS-DME have three weak hydrogen bonds: C-H---Y (Y = O, S, and F) Attention is paid to the roles of these weak hydrogen bonds in the supramolecules and related biomolecules.

Intermolecular Interactions (2) We have been applying FTMW spectroscopy in a systematic way to complexes consisting of molecules which are playing important roles in the Earth’s atmosphere. We focus attention on the differences in the roles of the O and S atoms in the intermolecular interactions. CO CO 2 Ar Dimethyl ether (DME) Dimethyl sulfide (DMS) Ethylene oxide (EO) Ethylene sulfide (ES) Y. Kawashima et. al. J. Phys. Chem. 116 (2012) 1224 We concluded that the C atom of the CO 2 in CO 2 -EO and CO 2 -ES is bound to the lone pair of O of the EO or of S of the ES. X

(a) DMS-DME H 2 CO-DMS H 2 CO-DME c τ θ a a c τ θ Our aim is to obtain information on intramolecular interactions in the H 2 CO-DME and H 2 CO-DMS.

Conformer I (0 cm -1 ) (τ = 90 , θ = 90  ) Conformer V (979 cm -1 ) Stable conformers of the H 2 CO-DME and H 2 CO-DMS Optimized at MP2/ G(d, p) Conformer I (0 cm -1 ) (τ = 90 , θ = 90  ) Conformer III (285 cm -1 ) (τ = 0 , θ = 90  ) Conformer II (160 cm -1 ) (τ = 90 , θ = 0  ) Conformer IV (381 cm -1 ) (τ = 0 , θ = 0  ) Conformer V (1083 cm -1 ) Conformer II (709 cm -1 ) transition state (τ = 90 , θ = 0  ) Conformer IV (640 cm -1 ) (τ = 0 , θ = 30  ) Conformer III (365 cm -1 ) (τ = 0 , θ = 90  ) H 2 CO-DME H 2 CO-DMS

buffer gas heated nozzle 70 ℃ Heated nozzle: 70  C Sample ： paraformaldehyde(99%) Backing pressure : 3.0 atm Carrier gas : 0.5%DME diluted with Ar 0.5%DMS diluted with Ar Averaging ： 30  1000 shots Frequency region : 9  20 GHz Step ： 0.25 MHz Fourier transform microwave (FTMW) spectrometer

DME monomer H 2 CO-DME Ar-DME dimer DME dimerH 2 CO dimer Ar- H 2 CO H 2 CO monomer Observed spectra of the sample; H 2 CO and 0.5%DME/Ar J = 2 ← 1 a-type transitions J = 3 ← 2 a-type transitions frequency /MHz

Observed spectra of the H 2 CO-DME, D 2 CO-DME, H 2 CO-DMS, and D 2 CO-DMS Small splitting was observed for the H 2 CO-DME complex. MHz Doppler doublet

Molecular constants of the H 2 CO-DME, D 2 CO-DME, H 2 CO-DMS, and D 2 CO-DMS a H 2 CO-DME D 2 CO-DME H 2 CO-DMS D 2 CO-DMS (l) (h) A /MHz (42) (54) (70) (18) (30) B /MHz (27) (21) (25) (11) (31) C /MHz (23) (23) (24) (12) (26) Δ J /kHz (32) (28) (34) (14) (24) Δ JK /kHz (10) (26) (30) (51) (70) Δ K /kHz (39) (44) (60) (11) (17) δ J /kHz (19) (15) (19) (81) (47) δ K /kHz (69) (58) (56) (25) (40) Φ J /Hz (49) Φ JK /Hz (33) (38) σ b /kHz N (a-type) c / N (c-type) c / a The number in parentheses denotes 1σ. b Standard deviations. c Number of fitting transitions. Only H 2 CO-DME complex shows very small splittings.

Planar moment of inertia, P bb Comparison of observed and expected P bb values (in uÅ 2 ) Complex observed expected H 2 CO-DME D 2 CO-DME H 2 CO-DMS D 2 CO-DMS P bb (complex) = P aa (DME/DMS) + P bb (H 2 CO/D 2 CO) Planar moments of inertia of monomers (in uÅ 2 ) P aa P bb DME DMS H 2 CO D 2 CO b c c a It is concluded that the H 2 CO-DME and H 2 CO-DMS complexes are of C s symmetry. a c

Comparison of the experimental and ab initio calculated molecular parameters of the H 2 CO-DME and H 2 CO-DMS Parameter H 2 CO-DME H 2 CO-DMS Obsdab initio a Obsdab initio a A / MHz (6) (18) B / MHz (2) (12) C / MHz (2) (12) R cm / Å θ 1 / ° θ 2 / ° r(O--C) / Å r(S--C) / Å r 1 / Å r 2 / Å a Optimized at MP2/ G(d, p) r1r1 r2r2 r1r1 r2r2 θ1θ1 θ1θ1 θ2θ2 θ2θ2 Van der Waals radii O : 1.40 Å S : 1.85 Å C : 1.70 Å r(O--C) = 3.10 Å r(S--C) = 3.55 Å

Obtained bond distance R cm, estimated force constants k s, binding energy E B, corrected dissociation energy D % CP, calculated stabilized energy of the charge transfer CT of the H 2 CO-DME, H 2 CO-DMS, and related complexes ParameterR cm k s E B D 0 +50%CP CT A-B complex Ar-DME CO-DME CO 2 -DME DFE-DME H 2 CO-DME DME-DME DMS-DME TFE-DME IPOH a -DME Ar-DMS CO-DMS H 2 CO-DMS H 2 CO-H 2 CO a IPOH denotes iso-propanol. contributions of the CT A → BB → A NBO analysis with its implications for molecular structure and the stabilization energy through CT of the H 2 CO-DME and H 2 CO-DMS Ionization potential I p (DME) = eV I p (DMS) = 8.69 eV

Relationship between the binding energy E B and the stabilized energy of the charge transfer CT of the H 2 CO-DME, H 2 CO-DMS, and the related complexes. The red line shows that CT for H 2 CO-DME, H 2 CO-DMS, and (H 2 CO) 2 are about five times larger than E B. The blue line indicates the good correlation between the complexes which have small contributions of the CT.

V(τ) and V(θ) of the H 2 CO-DME and H 2 CO-DMS against τ and θ It is not easy to explain the observed small splittings of the H 2 CO-DME. cm -1 τ or θ /  V(τ)V(τ) V(τ)V(τ) V(θ)V(θ) V(θ)V(θ) V(θ)V(θ) V(θ)V(θ) H 2 CO-DME H 2 CO-DMS

summary 1. H 2 CO-DME and H 2 CO-DMS complexes are detected using FTMW spectroscopy. 2. The binding energy of the H 2 CO-DMS is larger than that of the H 2 CO-DME. 3. DME is strongly hydrogen bonded to other molecule, while H 2 CO behaves as a strong electron acceptor. The ionization potentials of the DME and DMS are 10.0 and 8.69 eV, respectively, therefore CT is larger in the H 2 CO-DMS than in the H 2 CO-DME. R cm = Å R cm = Å E B = 5.2 kJ/mol CT = 27.9 kJ/mol E B = 6.7 kJ/mol CT = 35.4 kJ/mol H 2 CO-DME H 2 CO-DMS