add a 'X' atom at the center of benzene, and link the "X" atom with two C atoms in para position(C1, C4), then add Cl atom to the "X" atom, adjust the C1-X-Cl angle to 90 degree, then adjust dihydral C1-X-C2-Cl to 90 or -90 degree.  

Thanks  Dr. Xiang Zhang. But the problem is that I have to measure the distance from the centroid of the ring to the Cl atom.

calculate the coortinates of the center of a benzene.Xcenter= (X1+X2+X3+X4+X5+x6)/6, Ycenter=........, Zcenter=..........; then Distance=Sqrt((Xcenter-Xcl)**2+(Ycenter-Ycl)**2+(Zcenter-Zcl)**2) 

add an X at the center of benzene in GView, (Dc1-X =  Dc4-X, Dc2-X = Dc5-X, Dc3-X = Dc6-X, after many adjustments， X will be very colse to ther center.), then directly measure the distance of X-Cl. 

Dear Parag,

Dr Xiang Zhang has already given the best answer,I only want to add something for your next question.In gaussview if you open a structure,press right-click go to edit and press "reorient" origin of the molecule is reset to center of molecule.You can cross check it by  right clicking at the view window>view>cartesian axes.place a ghost atom(X) at the centre (as Dr. Zhang said) and measure the distance.

Thanks Dr Sengupta, Now I understand Dr Zhang answer completly.

How do you draw from the center of a benzene molecule to some distant cation like Li+, Na+, K+, etc in Avogadro?

How do we calculate tilt angle ϕ that forms the Mn–O(water) and O(water)–CH vectors, where CH represents the centroid of the line connecting the two H atoms of the coordinated water molecule. A ϕ angle of 90° therefore indicates that the bisector of the coordinated water molecule is perpendicular to the Mn–O(water)

vector, while for a ϕ angle of 180°, the water molecule and the metal ion are on the same plane.In gaussian how we can do that?