The Mid-Infrared Spectroscopy Of Formic Acid Dimer

Formic acid dimer (FAD) is the smallest organic complex with a double hydrogen bond and serves as a prototype for hydrogen multiple proton transfer and has attracted the continued interest of spectroscopic and theoretical groups.We report the measurement of the IR high resolution spectrum of the C-O-H bending of deuterated formic acid dimer (HCOOD)2 in the region of 1383-1392.5cm-1 for the first time.The dimers were aggregated in a supersonic jet expansion and the measurements were carried out using our quantum cascade lasers.With the help of PGOPHER software,we were able to assign 425 rovibrational transitions including two bands, in which 221 transitions belong to (HCOOH-HCOOD) dimer and 204 transitons belong to (HCOOD)2.All of them are a-type transition.The vibrational frequency is determined to be 1386.7559cm-1 and 1391.0843cm-1 respectively.Within our experimental frequency resolution,we were not able to resolve the proton transfer tunneling splitting,which yields an upper limit of 0.0006cm-1 for the tunneling splitting of (HCOOD)2.Having compared the tunneling splitting of the various isotopomers including:(HCOOH)2,(DCOOH)2,and (DCOOD)2,we find out that the value of splitting will decrease vastly when D is substituted for H in the hydrogen bond.In addition,we have systematiclly compared the rotational constants of all the isotopomers,and this allowed us to deduce information on the structure of formic acid dimer.We realise that when H in the hydrogen bond was substituted by D,the length of hydrogen bond will increase slightly,which in line with C.Costain.Combination with the model of T.Neuheuser,we caculated the length of hydrogen bond and the result is 2.6837 A for the ground state and 2.6935 A for the excited state,which is in good agreement with A.Gutberlet's optimized result.