Entanglement Operator for a Multi-Qubit System

Suppose there is a linear chain molecule with three qubits for liquid state NMR quantum computing and we want to apply a two-qubit gate to qubits 2 and 3. A standard practice to implement such a selective gate is to use refocusing, in which a pair of 'hard' π-pulses is applied to qubit 1. These pulses, however, often bring about an unwanted effect on qubits 2 and 3 when the molecule is homonuclear. This phenomenon is known as the Bloch-Siegert effect and compensation for this effect is required for correct gate operation.