Thirty-six stable complexes of formic acid with formaldehydes and thioformaldehydes were determined on the potential energy surface, in which the XCHOÁÁÁHCOOH complexes are found to be more stable than the XCHSÁÁÁHCOOH counterparts, with X = H, F, Cl, Br, CH 3 , NH 2 . All complexes are stabilized by hydrogen bonds, and their contribution to the total stabilization energy of the complexes increases in going from C-HÁÁÁS to C-HÁÁÁO to O-HÁÁÁS and finally to O-HÁÁÁO. Remarkably, a significant blueshift of C sp2 -H bond by 81-96 cm −1 in the C sp2 -HÁÁÁO hydrogen bond has hardly ever been reported, and a considerable redshift of O-H stretching frequency by 206-544 cm −1 in the O-HÁÁÁO/S hydrogen bonds is also predicted. The obtained results in our present work and previous literatures support that a distance contraction and a stretching frequency blueshift of C-H bond involving hydrogen bond depend mainly on its polarity and gas phase basicity of proton acceptor, besides the rearrangement of electron density due to complex formation. Markedly, we suggest the ratio of deprotonation enthalpy to proton affinity (R c ) as an indicator to prospect for classification of hydrogen bonds. The symmetry adapted perturbation theory results show a larger role of attractive electrostatic term in XO-n as compared to that in XS-n and the electrostatic interaction is overwhelming dispersion or induction counterparts in stabilizing XO-n and XS-n, with n = 1, 2, 3.

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Two new dimeric monoterpene-linked coumarin glucosides, paratrimerins A (1) and B (2), and three known coumarins, 6-(6-hydroxy-3,7-dimethylocta-2,7-dienyl)-7-hydroxycoumarin (3), ostruthin (4), and ninhvanin (5), were isolated from the roots and stems of Paramignya trimera (OLIV.) GUILL. Key words Paramignya trimera; paratrimerin A; paratrimerin B; monoterpene-linked coumarin glycoside; RutaceaeParamignya is a genus (ca. 28 species) 1) belonging to the Rutaceae family. Since the 1990 s, phytochemical studies have focused on two species, P. monophylla and P. griffithii. While flavanones, triterpenoid, and chromene derivatives have been isolated from the stems of P. griffithii, 2) tirucallane-triterpene and several new coumarins have been found in P. monophylla fruits 3) and stem bark. 4) In Vietnam, seven Paramignya species have been identified, distributed mostly in the South of the country. 5) Paramignya trimera (OLIV.) GUILL., local name "Than duoc" or "Xao tam phan," is an endemic plant in South Vietnam. The stems and roots of the plant have been used in folk medicine to treat liver disease, especially cirrhotic ascites. 6) In the search for new natural compounds with biological activities relative to the treatment of liver disease, we isolated and structurally elucidated two new dimeric coumarin glycosides, namely paratrimerins A (1) and B (2), along with three known coumarins, 6-(6-hydroxy-3,7-dimethylocta-2,7-dienyl)-7-hydroxycoumarin (3), ostruthin (4), and ninhvanin (5), from the roots and stems of P. trimera. Although monoterpenelinked biscoumarins have recently been isolated from rutaceous plants, [7][8][9] compounds 1 and 2 are the first monoterpenelinked biscoumarin glycosides isolated from nature. Results and Discussion

Two new dimeric monoterpene-linked coumarin glucosides, paratrimerins A (1) and B (2), and three known coumarins, 6-(6-hydroxy-3,7-dimethylocta-2,7-dienyl)-7-hydroxycoumarin (3), ostruthin (4), and ninhvanin (5), were isolated from the roots and stems of Paramignya trimera (OLIV.) GUILL. collected in Khanh Hoa province, Vietnam. Compound 1 comprises two 7-O-β-D-glucopyranoside coumarins linked at positions 6,6 via a 1,3,4,4-tetrasubstituted cyclohexene containing a monoterpene bridge, whereas compound 2 is a β-D-apiofuranosyl(1→6)-β-D-glucopyranosyl derivative of 1. The chemical structures of these compounds were determined by one dimensional (1D) and 2D-NMR and high resolution-electrospray ionization (HR-ESI)-MS spectroscopy. Paramignya is a genus (ca. 28 species) 1) belonging to the Rutaceae family. Since the 1990 s, phytochemical studies have focused on two species, P. monophylla and P. griffithii. While flavanones, triterpenoid, and chromene derivatives have been isolated from the stems of P. griffithii, 2) tirucallane-triterpene and several new coumarins have been found in P. monophylla fruits 3) and stem bark. 4) In Vietnam, seven Paramignya species have been identified, distributed mostly in the South of the country. 5) Paramignya trimera (OLIV.) GUILL., local name "Than duoc" or "Xao tam phan," is an endemic plant in South Vietnam. The stems and roots of the plant have been used in folk medicine to treat liver disease, especially cirrhotic asci-tes. 6) In the search for new natural compounds with biological activities relative to the treatment of liver disease, we isolated and structurally elucidated two new dimeric coumarin glyco-sides, namely paratrimerins A (1) and B (2), along with three known coumarins, 6-(6-hydroxy-3,7-dimethylocta-2,7-dienyl)-7-hydroxycoumarin (3), ostruthin (4), and ninhvanin (5), from the roots and stems of P. trimera. Although monoterpene-linked biscoumarins have recently been isolated from ruta-ceous plants, 7-9) compounds 1 and 2 are the first monoterpene-linked biscoumarin glycosides isolated from nature. Results and Discussion Compound 1 was obtained as a white amorphous powder-with a negative optical rotation ([α] D 25 −25.0°, c=0.02, MeOH). Its molecular formula was determined to be C 40 H 44 O 16 based on the quasi-molecular ion peak observed at m/z 781.2691 [M+H] + (Calcd for C 40 H 45 O 16 ± , 781.2702) and m/z 803.2548 [M+Na] + (Calcd for C 40 H 44 O 16 Na ± , 803.2522) in the positive high resolution-electrospray ionization (HR-ESI)-MS spectrum. A further fragment ion was identified at m/z 457.1637 ([M−2×C 6 H 10 O 5 +H] + , Calcd for C 28 H 25 O 6 ±

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