Mrs/Miss Antony ,

You can use GaussView.

Thanks B. Ivanova.

I  used gauss view only.  But found very difficult. can't connect to the exact position where we want .

Haven't use Gaussview for a while, but I know at least in Avogadro you can translate/rotate each set of rings separately.

That should give you freedom to place your molecule in any orientation towards the other.

in gauss view:

1) open both of your molecules in separate windows, copy benzene by edit-->copy

2) in the window of biphenyl ring, past your molecule by Edit-->past--> append molecule

3) to set the position correctly you should use keyboard and mouse

holding (shift+alt) will move only that molecule which you click.

holding (Alt) and moving mouse will rotate only that molecule which you click.

4) move and rotate your molecule using keyboard shortcuts and place your molecule at your desired location make/remove bond by builder--> modify bond and then clean the structure (Edit --->clean)

5) now you can optimize. after which you get the desired structure

By using the option 'modify bond' as shown as an attachment.

1. You should place your discrete molecules in a window;

2. You should press the symbol corresponding to the 'modify bond' (step 2, attachment)

3. You should select atoms (Step 3, attachment)

3. When the new window is opened you can specify the bond length (step 4, attachment)

Thank you Fernando.V, Antarip .H, B. Ivanova and Keshav Singh.. for the valuable suggestions. it really works so well. Ya I could attach two molecules by following your instructions.