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Sohn, Woon Yong,Ishiuchi, Shun-ichi,Miyazaki, Mitsuhiko,Kang, Jeongseok,Lee, Soyoung,Min, Ahreum,Choi, Myong Yong,Kang, Hyuk,Fujii, Masaaki The Royal Society of Chemistry 2013 Physical chemistry chemical physics Vol.15 No.3
<P>Electronic and vibrational spectra of acetaminophen were measured by using UV-UV hole burning (HB) and IR dip spectroscopy. HB spectra show the coexistence of 4 different species, which include two new ones. Low-frequency transitions in the spectra are reproduced by a one-dimensional periodic potential with a free-rotor basis set for the methyl group. From the analysis, we concluded that acetaminophen has two conformers and each conformer gives two independent transitions starting from the most stable 0a<SUB>1</SUB> and the hot 1e internal rotational levels. It is also found that the HB spectrum of the <I>trans</I>-conformer in the previous report is that from the 1e excited level, while the HB spectrum of the <I>cis</I>-conformer is contaminated by the transitions of the <I>trans</I>-conformer. Potential curves of the methyl rotational motion are determined both in S<SUB>0</SUB> and S<SUB>1</SUB>.</P> <P>Graphic Abstract</P><P>Four species of acetaminophen were found and assigned to two isomers with methyl internal rotations. <IMG SRC='http://pubs.rsc.org/services/images/RSCpubs.ePlatform.Service.FreeContent.ImageService.svc/ImageService/image/GA?id=c2cp43552g'> </P>
Sohn, Woon Yong,Kim, Jeong Jin,Jeon, Myeongwon,Aoki, Takuma,Ishiuchi, Shun-ichi,Fujii, Masaaki,Kang, Hyuk The Royal Society of Chemistry 2018 Physical chemistry chemical physics Vol.20 No.30
<P>UV-UV hole burning and IR dip spectra of l-β<SUP>3</SUP>-homotryptophan were measured by a laser desorption supersonic jet technique as a bottom-up approach to understand the secondary structures of β-peptides. 14 conformers were found by UV-UV hole burning spectroscopy. The conformers were classified into three groups depending on their hydrogen bonding patterns observed in their conformer-specific IR spectra, and tentatively assigned by comparing with quantum chemical calculations. Group 1 had free OH stretch but no NH2 anti-symmetric stretch vibrational transition and was assigned to NH-π hydrogen bonded structures. Group 2, including the most abundant conformer, showed both free OH and NH2 anti-symmetric stretch vibrations, and belonged to NH-O hydrogen bonded conformations. Group 3 of conformers had hydrogen-bonded OH stretch IR transition and had OH-N hydrogen bonds. The internal hydrogen bond of group 3 is a C6 hydrogen bond due to the additional carbon atom at the β position and shows a shorter bond length than that of a C5 hydrogen bond. While the OH-N C6 hydrogen bond is stronger than NH-O, the entropic effect prefers the more flexible NH-O hydrogen bonded structure. It is expected that the unnatural C6 hydrogen bond influences the conformations of β-peptides and builds totally different secondary structures than those of α-peptides.</P>