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Mark Edgar
m.edgar@lboro.ac.uk

I have used Gaussian-03 to calculate the optimised structures and enegry difference between the trans and gauche rotamers of 1,2-difluoro-ethane using RHF and a variety of basis sets. My results match the data published in the paper by Wiberg etal Journal Americal Chemical Society, Vol 94, No 18, p6956, (1990) , where the favoured rotamer is the trans form for "simple" basis sets and the gauche form for "complex" basis sets. Experimental results show that the gauche form is lowest in energy.

Calculating the same gauche/trans structures and energies using Firefly have produced rather different results (I am still learning and trust that there is an error in my Firefly input file).

All help and guidance will be welcome.
Thanks in advance to anyone that can help.

Regards,

Mark



INPUT DATA FILES (trans and gaughe)

!                        
! F-CH2-CH2-F Trans
!                  
$CONTRL SCFTYP=RHF RUNTYP=OPTIMIZE MAXIT=900 COORD=ZMT nprint=-9 $END
$CONTRL ICUT=11 INTTYP=HONDO $END
$SYSTEM TIMLIM=600 MEMORY=9900000 SPLITF=.T. $END
$statpt nstep=900 $END
$p2p p2p=.t. dlb=.t. XDLB=.T. $END
$SMP CSMTX=.T. $END
$SCF    DIRSCF=.True.  NCONV=6 $END              
$GUESS  GUESS=HUCKEL   $END        
$BASIS  GBASIS=N31 NGAUSS=6 NDFUNC=1 NPFUNC=0 $END              
$ELMOM  IEMOM=2 WHERE=COMASS $END        
$DATA    
RHF/6-31G* F-CH2-CH2-F Trans  
CN 1  

F
C  1   B1  
C  2   B2    1    A1  
F  3   B1    2    A1    1   180.0
H  2   B3    1    A2    3   D1
H  2   B3    1    A2    3   -D1  
H  3   B3    4    A2    2   D1
H  3   B3    4    A2    2   -D1

B1 1.4
B2 1.51
B3 1.07
A1 107.0
A2 109.5
D1 119.9
$END




!                        
! F-CH2-CH2-F Gauche
!                  
$CONTRL SCFTYP=RHF RUNTYP=OPTIMIZE MAXIT=900 COORD=ZMT nprint=-9 $END
$CONTRL ICUT=11 INTTYP=HONDO $END
$SYSTEM TIMLIM=600 MEMORY=9900000 SPLITF=.T. $END
$statpt nstep=900 $END
$p2p p2p=.t. dlb=.t. XDLB=.T. $END
$SMP CSMTX=.T. $END
$SCF    DIRSCF=.True.  NCONV=6 $END              
$GUESS  GUESS=HUCKEL   $END        
$BASIS  GBASIS=N31 NGAUSS=6 NDFUNC=1 NPFUNC=0 $END              
$ELMOM  IEMOM=2 WHERE=COMASS $END        
$DATA    
RHF/6-31G* F-CH2-CH2-F Gauche  
CN 1  

F
C    1    B1
C    2    B2    1    A1
F    3    B1    2    A1    1    D3
H    2    B3    3    A2    4   -D1
H    3    B3    2    A2    1   -D1
H    2    B4    3    A3    4   D2
H    3    B4    2    A3    1   D2

B1  1.5
B2  1.5
B3  1.07
B4  1.07
A1  109.0
A2  109.0
A3  109.0
D1  60.0
D2  180.0
D3 70.0
$END