Context : Hot cores in molecular clouds , such as Orion KL , Sgr B2 , W51 e1/e2 , are characterized by the presence of molecules at temperature high enough to significantly populate their low-frequency vibrationally excited states .
For complex organic molecules , characterized by a dense spectrum both in the ground state and in the excited states , such as methyl formate , ethyl cyanide or dimethyl ether , lines from vibrationally excited states certainly participate to the spectral confusion .

Aims : Thanks to the recent laboratory study of the first torsional excited mode of methyl formate , we have searched for methyl formate , HCOOCH _ { 3 } , in its first torsionally excited state ( \upsilon _ { t } =1 ) in the molecular cloud W51 e2 .

Methods : We have performed observations of the molecular cloud W51 e2 in different spectral regions at 1.3 , 2 and 3 mm with the IRAM 30 m single dish antenna .

Results : Methyl formate in its first torsionally excited state ( \upsilon _ { t } =1 at 131 cm ^ { -1 } ) is detected for the first time toward W51 e2 .
82 transitions have been detected among which 46 are unblended with other species .
For a total of 16 A-E pairs in the observed spectrum , 9 are unblended ; these 9 pairs are all detected .
All transitions from excited methyl formate within the observed spectral range are actually detected and no strong lines are missing .
The column density of the excited state is comparable to that of the ground state .
For a source size of 7″we find that \mathrm { T _ { rot } } = 104 \pm 14 K and N = 9.4 ^ { +4.0 } _ { -2.8 } \times 10 ^ { 16 } cm ^ { -2 } for the excited state and \mathrm { T _ { rot } } = 176 \pm 24 K and N = 1.7 ^ { + .2 } _ { - .2 } \times 10 ^ { 17 } cm ^ { -2 } for the ground state .
Lines from ethyl cyanide in its two first excited states ( \upsilon _ { t } =1 , torsion mode at 212 cm ^ { -1 } ) and ( \upsilon _ { b } =1 , CCN in-plane bending mode at 206 cm ^ { -1 } ) are also present in the observed spectrum .
However blending problems prevent a precise estimate of its abundance .
However as for methyl formate it should be comparable with the ground state for which we find \mathrm { T _ { rot } } = 103 \pm 9 K and N = 3.7 ^ { +0.6 } _ { -0.4 } \times 10 ^ { 15 } cm ^ { -2 } for a 7″source size .

Conclusions : With regard to the number of lines of excited methyl formate and ethyl cyanide detected in W51 e2 , it appears that excited states of large molecules certainly account for a large number of unidentified lines in spectral survey of molecular clouds .