The Washington Capitals scored four goals on four power plays last night against the best penalty-killing unit in the National Hockey League. They never got a fifth chance, however, despite considerable provocation, and at equal strength they were no match for the Boston Bruins.

Boston, the best road club in the NHL, tied a Capital Centre record with six goals in the second period as it wiped out a two-goal deficit and thrashed the Capitals, 7-4. Rick Middleton netted three of the goals for his fourth career hat trick.

Adding to Washington's woes was an injury to winger Bengt Gustafsson, who left in the second period with what was tentatively diagnosed as ligament damage to his right ankle. Gustafsson banged Boston's Ray Bourque against the boards and Bourque fell on Gustafsson, whose legs became tangled.

Washington Coach Bryan Murray was so disgusted with referee Denis Morel's officiating that he stood and applauded when Morel called a tripping penalty on Boston's Mike O'Connell with 2:21 left. Murray immediately received a bench minor.

"I kept sending Ryan Walter and Dennis Maruk out to ask him if interference was still in the rule book, but he just said it was a 7-3 hockey game and not to worry about it," Murray said. "So I got a little sarcastic and he was waiting."

When Morel did signal Boston fouls, the Capitals took full advantage. Walter scored twice in the first period to hit the 20-goal mark for the season. Ten have come on power plays, a vast change from a year ago, when he had four extra-man scores in his total of 24.

Torrie Robertson added a third goal on his only shift of the period to contribute to a 3-1 Washington lead at the intermission. Mike Gartner concluded the scoring in the final period and Maruk boosted his point total to 54 by assisting on all four goals.

The Capitals had a couple of chances to pad their lead early in the second period. Greg Theberge fired a shot that struck the mesh just outside the left post and caromed straight out, leaving Theberge and many of the 13,259 watching with the belief it had been in the net.

Then Bobby Carpenter, with Walter open on a two-on-one break, fired the puck into goalie Marco Baron instead of passing. The Bruins broke out quickly and, with Theberge falling, Middleton had a clean breakaway for the goal that made it 3-2.

"My shot hit the side of the mesh at a funny angle and came popping right out," Theberge said. "I thought it was a goal, but when neither the goal judge nor referee did anything I realized it wasn't in.

"Middleton ran a little bit of interference on me at the blueline. He was behind me, waiting for a breakaway pass, and he tapped me on the leg with his stick. I went down and he was gone. They were running interference all night and getting away with it."

Moments later, Maruk carried the puck on a two on one with Chris Valentine. Again, there was a shot instead of a pass, without result.

Boston pulled even on a 50-footer by Brad Park, after Keith Crowder dug the puck out behind the goal line and aimed a pass for the slot that instead wound up on Park's stick just inside the blueline.

When Middleton was allowed to walk in unmolested from the left-wing corner for the tie-breaking goal, it was obvious the Capitals were collapsing.

"Middleton made a great play coming out of the corner, but he should not have been allowed to do that unchecked," Murray said. "Obviously, our defense played awful bad tonight."

Don Marcotte converted a two on one with Steve Kasper for his first goal of the season to boost Boston's lead to 5-3. Then Paul MacKinnon, distracted by a stick from behind, lost the puck to Middleton, who completed his hat trick for a 6-3 margin.

Ray Bourque made it 7-3 while most of the crowd was watching a battle between Carpenter and Boston's Barry Pederson. Pederson hooked Carpenter, who turned and swung his stick like an ax, triggering some solid punches by both.

Boston's road record is 9-4-2 and the Bruins trail first-place Montreal by only one point in the Patrick Division. The Capitals, meanwhile, dropped eight points behind the New York Rangers, who will come to Landover Saturday night.

"We have to play at a hell of a high level to stay even with the Boston Bruins, the Chicago Black Hawks, the Montreal Canadiens or the New York Islanders," Murray said. "We have some people who aren't going to handle that at times very well. Every shift you have to burn yourself to the limit or they're going to take advantage of you."