To constrain a bond, you will need to enter in a section entitled "coord" below your cartesian coordinates or z-matrix. The section will find the lowest energy for your system while keeping that constrained bond distances the same.
An example...
blah
blah
blah
&zmat
blah
&
&coord
Ti C1 # 2.0
&
&guess
blah
The general format for constraining a bond between two atoms in the "coord" section is:
Atom1 Atom2 # distance
You can constrain more bonds by entering in more lines of the same format in the section.
CS